[media] v4l: rcar-fcp: Don't force users to check for disabled FCP support

The rcar_fcp_enable() function immediately returns successfully when the
FCP device pointer is NULL to avoid forcing the users to check the FCP
device manually before every call. However, the stub version of the
function used when the FCP driver is disabled returns -ENOSYS
unconditionally, resulting in a different API contract for the two
versions of the function.

As a user that requires FCP support will fail at probe time when calling
rcar_fcp_get() if the FCP driver is disabled, the stub version of the
rcar_fcp_enable() function will only be called with a NULL FCP device.
We can thus return 0 unconditionally to align the behaviour with the
normal version of the function.

Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>