[media] em28xx: overhaul em28xx_capture_area_set()

- move the bit shifting of width+height values inside the function
- fix the debug message format and output values
- add comment about the size limit (e.g. EM277x supports >2MPix)
- make void, because error checking is incomplete and we never check the
  returned value (we would continue anyway)

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>