V4L/DVB (7316): pvrusb2: Handle ATSC video standard bits

The pvrusb2 driver dynamically generates an enumeration of support
video standard combinations based on which video standard bits are
set.  ATSC modes don't fall into this since they are by nature not
analog.  The pvrusb2 driver has been warning about an inability to
classify ATSC standards.  This change causes the classification
algorithm to ignore any ATSC standards (such things are better handled
elsewhere anyway).

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