V4L/DVB (5081): Pvrusb2: VIDIOC_G_TUNER cleanup

Clean up use of VIDIOC_G_TUNER; we now correctly gather info from all
the I2C client modules.  Also abide by V4L2_TUNER_CAP_LOW
appropriately.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>