mwifiex: correct TX byte count statistics

The skb is modified in sending the TX packet. Save the original
packet length to a variable so that we can get the correct
statistics update.

Signed-off-by: Ujjal Roy <royujjal@gmail.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>