mwifiex: usb: return an error if kmalloc fails

The current code returns success if kmalloc fails.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>