mwifiex: fix usb tx data payload offset issue

Commit 84b313b3 ("mwifiex: make tx packet 64 byte DMA aligned")
induced payload offset issue for USB interface.

There is no USB interface header for tx packets, so there's no need to
pull interface length while processing tx skb.

This patch fixes this issue.

Signed-off-by: Zhaoyang Liu <liuzy@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>