At the moment, the WWAN core provides wwan_port_txon/off() to implement
blocking writes. The tx() port operation should not block, instead
wwan_port_txon/off() should be called when the TX queue is full or has
free space again.

However, in some cases it is not straightforward to make use of that
functionality. For example, the RPMSG API used by rpmsg_wwan_ctrl.c
does not provide any way to be notified when the TX queue has space
again. Instead, it only provides the following operations:

  - rpmsg_send(): blocking write (wait until there is space)
  - rpmsg_trysend(): non-blocking write (return error if no space)
  - rpmsg_poll(): set poll flags depending on TX queue state

Generally that's totally sufficient for implementing a char device,
but it does not fit well to the currently provided WWAN port ops.

Most of the time, using the non-blocking rpmsg_trysend() in the
WWAN tx() port operation works just fine. However, with high-frequent
writes to the char device it is possible to trigger a situation
where this causes issues. For example, consider the following
(somewhat unrealistic) example:

 # dd if=/dev/zero bs=1000 of=/dev/wwan0qmi0
 dd: error writing '/dev/wwan0qmi0': Resource temporarily unavailable
 1+0 records out

This fails immediately after writing the first record. It's likely
only a matter of time until this triggers issues for some real application
(e.g. ModemManager sending a lot of large QMI packets).

The rpmsg_char device does not have this problem, because it uses
rpmsg_trysend() and rpmsg_poll() to support non-blocking operations.
Make it possible to use the same in the RPMSG WWAN driver by adding
two new optional wwan_port_ops:

  - tx_blocking(): send data blocking if allowed
  - tx_poll(): set additional TX poll flags

This integrates nicely with the RPMSG API and does not require
any change in existing WWAN drivers.

With these changes, the dd example above blocks instead of exiting
with an error.

Cc: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: David S. Miller <davem@davemloft.net>