V4L/DVB (6585): Convert em28xx to video_ioctl2

Uses the newer ioctl handler at videodev. This patch also cleans up some
bad logic at the driver and do CodingStyle and other cleanups at the
resulting driver.

Also, since VIDIOCMBUF were not working, the V4L1 compat code were removed.
The compat code will eventually be re-inserted, if we find a clean way for
implementing compatibility with the old API.
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>