ath9k: remove incompatible IBSS interface check in change_iface

'cfg80211: fix interface combinations' ensures that if an interface
type is not advertised by the driver in any of the interface combinations
(via ieee80211_iface_combination) then it shall be treated as a single
incompatible interface. if there are more than one interfaces present
and changing them to incompatible interface type is not possible.
These checks will be properly handled by cfg80211_change_iface ->
cfg80211_can_change_interface.

this patch is dependent on 'cfg80211: fix interface combinations'

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