V4L/DVB (5574): Pvrusb2: Improve handling of PAL-60 video standard

This patch originated with Servaas Vandenberghe <vdb128@picaros.org>
and has been further developed a bit (to preserve saa7115 behavior).

These changes allow for correct operation of PAL-60 video (Servaas
tested this against a PAL-B/G tuner with the video standard overridden
as a module option).

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