mwl8k: Handle watchdog event with highest prioriry

Currently, command path waits till all the tx host queues
are empty. Whenever watchdog event is raised, firmware
expects driver to destroy ampdu queues immediately.

This requires corresponding commands i.e.
mwl8k_cmd_get_watchdog_bitmap and mwl8k_destroy_ba to be
sent without waiting for the tx queues to be completely
empty.

Use "watchdog_event_pending" to ensure the above mentioned
two commands are sent down immediately.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>