V4L/DVB (7304): pvrusb2: add function pvr2_hdw_cmd_powerdown

Call pvr2_hdw_cmd_powerdown to power down the device

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