mac80211: remove duplicate check in ieee80211_rx_mgmt_beacon

Remove a duplicate check in ieee80211_rx_mgmt_beacon,
there is no need to make again the same check for the
IEEE80211_HW_PS_NULLFUNC_STACK twice; the two ifs can
be consolidated.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
[reword commit message & break long lines and also
 clean up variable]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>