mac80211: remove unnecessary num_mcast_sta check

Checking for num_mcast_sta in __ieee80211_request_smps_ap() is
unnecessary as sta list will be empty in this case anyway, so
the list iteration will just exit immediately. Since this isn't
a "hot" code path, it doesn't really matter, and the next patch
will redefine num_mcast_sta to make this check invalid.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
[change commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>