rt2x00: rt2800pci: don't use TXWI_DESC_SIZE directly

Different chipsets may use different TXWI descriptor
size. Instead of using a hardcoded value, use the
'queue->winfo_size' which holds the correct value for
a given device.

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>