b43: fix the wrong assignment of status.freq in b43_rx()

Use the right function to update frequency value.

If rx skb is probe response or beacon, the wrong frequency value can
cause problem that bss info can't be updated when it should be.

Cc: <stable@vger.kernel.org>
Fixes: 8318d78a ("cfg80211 API for channels/bitrates, mac80211
and driver conversion")
Signed-off-by: ZHAO Gang <gamerh2o@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>