V4L/DVB (10506): saa7134: move tuner init code to saa7134-cards

On certain devices, before opening a tuner, we need to open the tuner
gate via i2c.

This patch just moves the tuner probing code to the same place where
such i2c commands are handled, to make easier to fix this trouble on
later patches.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>