mwifiex: fix tx data_sent issue for usb interface

This patch fix missing tx data_sent flag update for usb interface.
Except USB interface, data_sent flag has been updated in specific
file such as sdio.c and pcie.c. So only USB interface type need check
when TX data completed.

Signed-off-by: Zhaoyang Liu <liuzy@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>