[media] pvrusb2: get rid of warning: no previous prototype

drivers/media/usb/pvrusb2/pvrusb2-v4l2.c:199:5: warning: no previous prototype for 'pvr2_s_std' [-Wmissing-prototypes]
drivers/media/usb/pvrusb2/pvrusb2-v4l2.c:368:5: warning: no previous prototype for 'pvr2_s_frequency' [-Wmissing-prototypes]

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