V4L/DVB (9301): pvrusb2: Keep MPEG PTSs from drifting away

(Mike Isely) This change was empirically figured out by Boris Dores
after empirically comparing against behavior in the Windows driver.

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