mac80211: fix a compiler warning

net/mac80211/mlme.c: In function 'ieee80211_sta_work':
net/mac80211/mlme.c:1981: warning: too many arguments for format

Introduced by commit 04ac3c0e
("mac80211: speed up AP probing using nullfunc frames").

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>