[PATCH] rt2x00: Make *_beacon_update static

Make rt61pci_beacon_update and rt73usb_beacon_update static,
they are only used inside their own source file and then only
for setting it as callback funtion for mac80211.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>