mwifiex: change debug dump issue since skb maybe null

This patch fixes semantic warning for debugging data dump feature.
Previous code is based on the assumption that skb is not null.
New change makes sure that we already have data buffer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhaoyang Liu <liuzy@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>