media: ov7670: Return the real error code

When devm_clk_get() fails the real error code should be propagated,
instead of always returning -EPROBE_DEFER.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>