media: tuners/it913x.c: fix missing error code

Fixes this smatch warning:

drivers/media/tuners/it913x.c:65 it913x_init() warn: missing error code 'ret'

Set ret to -EINVAL if the clock identifier was invalid.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>