rt2x00: Centralize rt2800 beacon writing.

The beacon writing functions of rt2800pci and rt2800usb are now identical.
Move them to rt2800lib to only have one central function.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>