V4L/DVB (7690): pvrusb2-dvb: Remove digital_up flag

Other pvrusb2-dvb changes have made the digital_up flag obsolete.  So
kill it.

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