mwifiex: remove unneeded kfree(NULL);

The previous if statement means this that pointer is NULL so there
is no need to free it.

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>