V4L/DVB (11793): xc5000: Properly support power down for newer firmware

Xceive got rid of the XREG_POWER_DOWN register in later firmware revisions.
Their technical support informed me that the correct way to put the tuner
to sleep is to pull the reset pin (but don't reload the firmware).

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>