Revert "mac80211: move "bufferable MMPDU" check to fix AP mode scan"

This reverts commit 277d916f as it was
at least breaking iwlwifi by setting the IEEE80211_TX_CTL_NO_PS_BUFFER
flag in all kinds of interface modes, not only for AP mode where it is
appropriate.

To avoid reintroducing the original problem, explicitly check for probe
request frames in the multicast buffering code.

Cc: stable@vger.kernel.org
Fixes: 277d916f ("mac80211: move "bufferable MMPDU" check to fix AP mode scan")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>