media: radio: Tuning bugfix for si470x over i2c

Fixed si470x_set_channel() trying to tune before chip is turned
on, which causes warnings in dmesg and when probing, makes driver
wait for 3s for tuning timeout. This issue did not affect USB
devices because they have a different probing sequence.

Signed-off-by: Douglas Fischer <fischerdouglasc@gmail.com>
[hans.verkuil@cisco.com: fixed space-after-( checkpatch warning]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>