ath9k: use new FIF_PSPOLL configure filter

We used to set pspoll filter on ath9k on AP mode but
we no longer need this mode specific check as mac80211
now does the check for us and informs us when we should
enable pspoll through FIF_PSPOLL.

Cc: Igor Perminov <igor.perminov@inbox.ru>
Cc: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>