mwifiex: prevent corruption instead of just warning

Probably we never hit this condition, but in case we do, we may as
well put a return here instead of just printing a warning message and
then corrupting memory.  The caller doesn't check the return code.

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>