mac80211: fix suppressing probe responses in ad-hoc mode

The commit "mac80211: reply to directed probes in IBSS" changed ad-hoc
specific code to respond to unicast probe requests, even if
drv_tx_last_beacon returns false, however due to confusion over the
meaning of the IEEE80211_RX_RA_MATCH flag, it also unconditionally
enabled responding to multicast probe requests.
Fix this by explicitly checking for a multicast destination address
instead.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>