mwifiex: add a kfree() to an error path

We're not likely to hit this small memory leak, but lets fix it
anyway to keep the static checkers happy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>