ath9k: Use configurable timeout for flush

The timeout value for flushing the TX queues
is hardcoded at 200ms right now. Use a channel
context-specific value instead to allow adjustments
to the timeout in case MCC is enabled.

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