wimax/i2400m: when stopping the device, cancel any pending message

The stop procedure for the device must make sure that any task that is
waiting on a message is properly cancelled.

This was being taken care of only by the __i2400m_dev_reset_handle()
path and the rest was working by chance because the waits have a
timeout.

Fixed by adding a proper cancellation in __i2400m_dev_stop().

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>