V4L/DVB: pvrusb2: Adjust 300msec digitizer wait to be more selective

We know that the 300msec settling time after starting the digitizer is
only really needed when the digitizer is a SAA7115.  So if we're not
using a SAA7115, skip the delay.

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