rt2x00: Limit beacon updates in bss_info_changed to USB devices

Currently there are two places that trigger a beacon update on PCI
devices. The bss_info_changed callback and the periodic update
triggered by the TBTT or PRETBTT interrupt.

Since the next TBTT or PRETBTT interrupt will periodically fetch an
updated beacon remove the update_beacon call in the bss_info_changed
callback for PCI devices.

In the worst case it will take one beacon interval longer to fetch
the new beacon then before. For devices that have a PRETBTT interrupt
there should be no change at all.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>