rt2x00: Make rt2800_write_beacon only export to GPL

rt2800_write_beacon is the only function which uses
EXPORT_SYMBOL instead of EXPORT_SYMBOL_GPL. All symbols
in rt2x00 should however use the GPL restricted export.

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