media: pvrusb2: correctly return V4L2_PIX_FMT_MPEG in enum_fmt

The pvrusb2 code appears to have a some old workaround code for xawtv that causes a
WARN() due to an unrecognized pixelformat 0 in v4l2_ioctl.c.

Since all other MPEG drivers fill this in correctly, it is a safe assumption that
this particular problem no longer exists.

While I'm at it, clean up the code a bit.

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