rt2x00: Make periodic beacon updates for PCI devices atomic

Allow the beacondone and pretbtt functions to update the beacon from
atomic context by using the beacon update functions with caller locking.

This is a preparation for moving the periodic beacon handling into
tasklets that require atomic context.

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>