[media] xc4000: implemented power management

The following patch implements the xc4000_sleep() function.
The 'no_powerdown' module parameter is now interpreted differently:
  - 0 uses a device-specific default
  - 1 disables power management like before
  - 2 enables power management

Signed-off-by: Istvan Varga <istvan_v@mailbox.hu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>