media: ov2680: Allow probing if link-frequencies is absent

Since commit 63b0cd30 ("media: ov2680: Add bus-cfg / endpoint
property verification") the ov2680 no longer probes on a imx7s-warp7:

ov2680 1-0036: error -EINVAL: supported link freq 330000000 not found
ov2680 1-0036: probe with driver ov2680 failed with error -22

As the 'link-frequencies' property is not mandatory, allow the probe
to succeed by skipping the link-frequency verification when the
property is absent.

Cc: stable@vger.kernel.org
Fixes: 63b0cd30 ("media: ov2680: Add bus-cfg / endpoint property verification")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>