carl9170: handle traps from firmware loader

This patch changes the way the driver deals with
command responses and traps which are sent through
the special interrupt input endpoint 3.

While the carl9170 firmware does not use this
endpoint for command responses or traps, the
firmware loader on the device does. It uses it
to notify the host about 'watchdog triggered'
in case the firmware/hardware has crashed.

Note:
Even without this patch, the driver is still
able to detect the mishap and reset the device.
But previously it did that because the trap
event caused an out-of-order message sequence
number error, which also triggered a reset.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>