mac80211: uAPSD - fix IEEE80211_FCTL_MOREDATA bit setting

Set IEEE80211_FCTL_MOREDATA bit also in case we have buffered
frames (more than one) only for one AC.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>