media: v4l: fwnode: Make v4l2_fwnode_endpoint_free() safer

Assign vep->link_frequencies to NULL after releasing its memory. Without
this change, multiple calls to v4l2_fwnode_endpoint_free() would result in
double kfree calls.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>