mac80211: close the SP when we enqueue frames during the SP

Since we enqueued the frame that was supposed to be sent
during the SP, and that frame may very well cary the
IEEE80211_TX_STATUS_EOSP bit, we may never close the SP
(WLAN_STA_SP will never be cleared). If that happens, we
will not open any new SP and will never respond to any poll
frame from the client.
Clear WLAN_STA_SP manually if a frame that was polled during
the SP is queued because of a starting A-MPDU session. The
client may not see the EOSP bit, but it will at least be
able to poll new frames in another SP.

Reported-by: Alesya Shapira <alesya.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
[remove erroneous comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>