rt2x00: rt2800: unify [RT]XWI_SIZE defines

Use common names instead of chip specific ones.
The patch contains no functional changes, but
it makes it easier to add support for further
descriptor sizes.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>