[media] v4l2-pci-skeleton.c: fix alternate field handling

For interlaced HDTV timings the correct field setting is FIELD_ALTERNATE,
not INTERLACED. Update this template driver accordingly:

- add check for the invalid combination of read() and FIELD_ALTERNATE.
- in the interrupt handler set v4l2_buffer field to alternating TOP and
  BOTTOM.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>