ath9k: Cleanup add/change_interface callbacks

*Remove all the checks that will be handled by cfg80211
based on the interface combination advertised. For instance,
driver supports at the maximum 8 beaconing interface, while
we advertise maximum 8 beaconing interface in the interface
combination support.

*cfg80211 will take care of not allowing
us to add an interface that is not supported by the
driver, further if the change_interface changes the
old interface to a beaconing interface while we had
reached the max limit of 8 beaconing interface, again
cfg80211 takes care of this stuff!
So remove all these checks.

*Beautify placing PS wrappers in the appropriate
position.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>