V4L/DVB (5089): Pvrusb2: V4L EXT_CTRLS fixup

Attempts to enumerate or operate on a group of EXT_CTRLS where the
group size is zero is OK; don't fail on such operations.  At least one
application uses this to probe for the existence of this API so let it
succeed.

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