ath9k: Improve flush() in mcc mode

The flush timeout in MCC mode is very small, since
we are constrained by the time slice for each
channel context, but since only the HW queues are
flushed when switching contexts, it is acceptable.

Since the SW queues are also emptied in the mac80211 flush()
callback, a larger duration is needed. Add an override
argument to __ath9k_flush() and set it when flush()
is called in MCC mode. This allows the driver to
drain both the SW and HW queues.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>