media: v4l2-pci-skeleton: replace vb2_buffer with vb2_v4l2_buffer

Commit 2d700715 ("[media] media: videobuf2: Restructure vb2_buffer")
replaced vb2_buffer with vb2_v4l2_buffer in all v4l2 drivers. The
restructuring skipped the v4l2-pci-skeleton, probably because it resides
outside the drivers directory.

The v4l2_buf_ops assume that the passed buffer is a vb2_v4l2_buffer.
This is not the case if the skel_buffer is based on vb2_buffer instead
of vb2_v4l2_buffer.

Replace vb2_buffer with vb2_v4l2_buffer in the skeleton to make sure
that future drivers that are based on the skeleton use vb2_v4l2_buffer.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>