Following a firmware update of the modem, the interface for the AT
command port changed, so add it back.

T:  Bus=08 Lev=01 Prnt=01 Port=01 Cnt=02 Dev#=  2 Spd=5000 MxCh= 0
D:  Ver= 3.20 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 9 #Cfgs=  1
P:  Vendor=1199 ProdID=90d3 Rev=00.06
S:  Manufacturer=Sierra Wireless, Incorporated
S:  Product=Sierra Wireless EM9191
S:  SerialNumber=xxxxxxxxxxxxxxxx
C:  #Ifs= 4 Cfg#= 1 Atr=a0 MxPwr=896mA
I:  If#=0x0 Alt= 0 #EPs= 1 Cls=02(commc) Sub=0e Prot=00 Driver=cdc_mbim
I:  If#=0x1 Alt= 1 #EPs= 2 Cls=0a(data ) Sub=00 Prot=02 Driver=cdc_mbim
I:  If#=0x3 Alt= 0 #EPs= 3 Cls=ff(vend.) Sub=ff Prot=40 Driver=(none)
I:  If#=0x4 Alt= 0 #EPs= 2 Cls=ff(vend.) Sub=ff Prot=30 Driver=option
Signed-off-by: Benoît Monin <benoit.monin@gmx.fr>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>