rt2x00usb: Zero USB padding before sending URB

When USB driver requires padding at the end of frame or URB it will report
this need by increasing return value of get_tx_data_len callback. Common
USB code uses that return value as desired URB length.

Ensure that appropriate part of skb's tailroom exists and is zeroed.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
--
 drivers/net/wireless/rt2x00/rt2x00usb.c |   16 +++++++++++++---
 1 files changed, 13 insertions(+), 3 deletions(-)
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>