ath9k: Fix MCC flush timeout

In MCC mode, the duration for a channel context
is half the beacon interval and having a large
flush timeout will adversely affect GO operation,
since the default value of 200ms will overshoot
the advertised NoA absence duration.

The scheduler initiates a channel context switch
only when the slot duration for the current
context expires, so there is no possibility of
having a fixed timeout for flush.

Since the channel_switch_time is added to the
absence duration when the GO sets up the NoA
attribute, this is the maximum time that we
have to flush the TX queues. The duration is very
small, but we don't have a choice in MCC mode.

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