[media] media: v4l2-pci-skeleton: Fix error handling path in 'skeleton_probe()'

If this memory allocation fails, we must release some resources, as
already done in the code below and above.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>