cfg80211: move cfg80211_exit to .exit.text

cfg80211_exit is only used as module_exit function, so it can go to
.exit.text saving a few bytes when CONFIG_CFG80211=y.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>