V4L/DVB (5443): Saa7134: put tuner to sleep mode after board initialization

Besides power saving, this puts the AGC output of the tda8290
to tristate. This is necessary for some hybrid boards which
don't use a multiplexer for the AGC

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>