mwl8k: Handle the watchdog event from the firmware

When an ampdu stream is on, if the firmware rate adaptation
logic decides that the outgoing packet rate to the station needs
to go below 6.5Mbps (non HT rate), it sends an event indicating that
the ampdu stream needs to be destroyed. Handle this event in the driver
and destroy the ampdu stream so that the rate can go below 6.5Mbps

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>