mwifiex: improve error path handling in usb.c

skb allocated during initialisation is reused for receiving
commands/events by USB interface. We miss to reset skb->data in
failure cases. This patch takes care of it.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>