V4L/DVB (7883): pvrusb2: make default frequency configurable via modprobe option

The pvrusb2 driver initially sets the tuner to known broadcast frequencies
in the Chicago area, to ease driver testing for the maintainer.

This patch keeps those default frequencies, but allows them to be altered
via modprobe option.  This allows the same ease and convenience for testing
multiple pvrusb2 devices one after another under other conditions and areas.

For instance, the default initial frequency, 175.25 MHz, might not
necessarily be valid on all cable television networks, but usually will be a
valid NTSC broadcast channel.

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