mac80211: fix dynamic power save for devices with nullfunc support in hw

In TX path it was assumed that dynamic power save works only if
IEEE80211_HW_PS_NULLFUNC_STACK is set. But is not the case, there are
devices which have nullfunc support in hardware but need mac80211
to handle dynamic power save timers, TI's wl1251 is one of them.

The fix is to not check for IEEE80211_HW_PS_NULLFUNC_STACK in
is_dynamic_ps_enabled(), instead check IEEE80211_HW_SUPPORTS_PS and
IEEE80211_HW_SUPPORTS_DYNAMIC_PS flags and act accordingly.

Tested with wl1251.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>