rt2x00: rt2800pci: move TX descriptor functions to the rt2800mmio module

Move the functions into a separate module, in order
to make those usable from other modules. Also move
the TX descriptor related defines from rt2800pci.h
into rt2800mmio.h.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>