V4L/DVB (6107): dvb_frontend: clean-up shutdown handling

Now dvb_powerdown_on_sleep controls whether
- whether LNB power is turned off
- whether the tuner/frontend enters sleep mode

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>