staging: rtl8712: freeing an ERR_PTR

If memdup_user() fails then "pparmbuf" is an error pointer and we can't
pass it to kfree().  I changed the "goto _r871x_mp_ioctl_hdl_exit" to a
direct return.

I changed the earlier goto to a direct return as well for consistency
and removed the "pparmbuf = NULL" initializer since it's no longer
needed.

Fixes: 45de4327 ('Staging: rtl8712: Use memdup_user() instead of copy_from_user()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>