[media] rcar_drif: fix potential uninitialized variable use

Older compilers like gcc-4.6 may run into a case that returns
an uninitialized variable from rcar_drif_enable_rx() if that
function was ever called with an empty cur_ch_mask:

drivers/media/platform/rcar_drif.c:658:2: error: ‘ret’ may be used uninitialized in this function [-Werror=uninitialized]

Newer compilers don't have that problem as they optimize the
'ret' variable away and just return zero in that case.

This changes the function to return -EINVAL for this particular
failure, to make it consistent across all compiler versions.
In case gcc gets changed to report a warning for it in the
future, it's also a good idea to shut it up now.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82203

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Ramesh Shanmugasundaram <ramesh.shanmugasundaram@bp.renesas.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/platform/rcar_drif.c

@@ -630,7 +630,7 @@ static int rcar_drif_enable_rx(struct rcar_drif_sdr *sdr)
 {
 	unsigned int i;
 	u32 ctr;
-	int ret;
+	int ret = -EINVAL;
 
 	/*
 	 * When both internal channels are enabled, they can be synchronized