ath9k: Add a config option for channel context

Channel contexts are supported only for P2P right now,
so make sure that the 'normal' path remains unaffected
by using a config option. This will also reduce the size
of the driver.

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