ath9k: Fix PS wrappers in ath9k_set_coverage_class

this callback is called during suspend/resume and also via iw command.
it configures parameters like sifs, slottime, acktimeout in
ath9k_hw_init_global_settings where few REG_READ, REG_RMW are also done
and hence the need for PS wrappers

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