mwifiex: cancel pending commands for signal

When a thread is interrupted by signal, all
wait_event_interruptible calls after queueing commands return
an error. Numbers of commands in pending queue are increased
in this case. Sometimes all commands nodes in pool are filled.

We will cancel pending commands when signal is received.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>