net/mac80211/rx.c: fix off-by-one

This patch fixes an off-by-one in net/mac80211/rx.c introduced by
commit 8318d78a


(cfg80211 API for channels/bitrates, mac80211 and driver conversion)
and spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>