wl1271: fix notifier interface supported test

The "(wl == NULL)" test doesn't work here because "wl" is always
non-null.  The intent of the code is to return if the interface
was not supported by the driver.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>