mac80211: reduce the number of retries for nullfunc probing

Since nullfunc frames are transmitted as unicast frames, they're more
reliable than the broadcast probe requests, so we need fewer retries
to figure out whether the AP is really gone.

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