V4L/DVB (6095): ivtv: fix VIDIOC_G_ENC_INDEX flag handling

Due to a documentation bug (the type mask is 3 bits long, not 2) the wrong
frame types were filled in: the B and P frame types were swapped.

This bug also hid a second bug: when a capture is stopped a last entry is
written into the pgm index buffer with internal type 0, denoting the end
of the program. This entry wasn't ignored, instead it was accidentally
returned to the caller as a P frame.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>