rt2x00: Push beacon TX descriptor writing to drivers.

Not all the devices require a TX descriptor to be written (i.e. rt2800
device don't require them). Push down the creation of the TX descriptor
to the device drivers so that they can decide for themselves whether
a TX descriptor is to be created.

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