Subclassing the generic USB device driver to override the
default configuration selection regardless of matching interface
drivers.

The r815x family devices expose a vendor specific function which
the r8152 interface driver wants to handle.  This is the preferred
device mode. Additionally one or more USB class functions are
usually supported for hosts lacking a vendor specific driver. The
choice is USB configuration based, with one alternate function per
configuration.

Example device with both NCM and ECM alternate cfgs:

T:  Bus=02 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  4 Spd=5000 MxCh= 0
D:  Ver= 3.20 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 9 #Cfgs=  3
P:  Vendor=0bda ProdID=8156 Rev=31.00
S:  Manufacturer=Realtek
S:  Product=USB 10/100/1G/2.5G LAN
S:  SerialNumber=001000001
C:* #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=256mA
I:* If#= 0 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=ff Prot=00 Driver=r8152
E:  Ad=81(I) Atr=02(Bulk) MxPS=1024 Ivl=0ms
E:  Ad=02(O) Atr=02(Bulk) MxPS=1024 Ivl=0ms
E:  Ad=83(I) Atr=03(Int.) MxPS=   2 Ivl=128ms
C:  #Ifs= 2 Cfg#= 2 Atr=a0 MxPwr=256mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=02(comm.) Sub=0d Prot=00 Driver=
E:  Ad=83(I) Atr=03(Int.) MxPS=  16 Ivl=128ms
I:  If#= 1 Alt= 0 #EPs= 0 Cls=0a(data ) Sub=00 Prot=01 Driver=
I:  If#= 1 Alt= 1 #EPs= 2 Cls=0a(data ) Sub=00 Prot=01 Driver=
E:  Ad=81(I) Atr=02(Bulk) MxPS=1024 Ivl=0ms
E:  Ad=02(O) Atr=02(Bulk) MxPS=1024 Ivl=0ms
C:  #Ifs= 2 Cfg#= 3 Atr=a0 MxPwr=256mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=02(comm.) Sub=06 Prot=00 Driver=
E:  Ad=83(I) Atr=03(Int.) MxPS=  16 Ivl=128ms
I:  If#= 1 Alt= 0 #EPs= 0 Cls=0a(data ) Sub=00 Prot=00 Driver=
I:  If#= 1 Alt= 1 #EPs= 2 Cls=0a(data ) Sub=00 Prot=00 Driver=
E:  Ad=81(I) Atr=02(Bulk) MxPS=1024 Ivl=0ms
E:  Ad=02(O) Atr=02(Bulk) MxPS=1024 Ivl=0ms

A problem with this is that Linux will prefer class functions over
vendor specific functions. Using the above example, Linux defaults
to cfg #2, running the device in a sub-optimal NCM mode.

Previously we've attempted to work around the problem by
blacklisting the devices in the ECM class driver "cdc_ether", and
matching on the ECM class function in the vendor specific interface
driver. The latter has been used to switch back to the vendor
specific configuration when the driver is probed for a class
function.

This workaround has several issues;
- class driver blacklists is additional maintanence cruft in an
  unrelated driver
- class driver blacklists prevents users from optionally running
  the devices in class mode
- each device needs double match entries in the vendor driver
- the initial probing as a class function slows down device
  discovery

Now these issues have become even worse with the introduction of
firmware supporting both NCM and ECM, where NCM ends up as the
default mode in Linux. To use the same workaround, we now have
to blacklist the devices in to two different class drivers and
add yet another match entry to the vendor specific driver.

This patch implements an alternative workaround strategy -
independent of the interface drivers.  It avoids adding a
blacklist to the cdc_ncm driver and will let us remove the
existing blacklist from the cdc_ether driver.

As an additional bonus, removing the blacklists allow users to
select one of the other device modes if wanted.
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>