rt2800usb: Let rt2x00usb handle USB padding

Older USB drivers does not append end padding to skb but instead report
it in size of data to be transmitted to HW. rt2800usb should follow that
behaviour. Custom write_tx_data callback which was adding pad to skb
is not be needed any more.

Thanks to this patch frames handed back from rt2800usb to mac80211 will
no longer contain end padding.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>