media: v4l2-pci-skeleton.c: fix doc warning

Document the 'field' field to fix this warning:

samples/v4l/v4l2-pci-skeleton.c:80: warning: Function parameter or member 'field' not described in 'skeleton'

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