rt2x00: rt2800usb: use correct [RT]XWI size for RT3593

The RT3593 chipset requires different [RT]XWI size
values. Modify the driver to use the correct values.

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>