media: radio: Critical interrupt bugfix for si470x over i2c

Fixed si470x_start() disabling the interrupt signal, causing tune
operations to never complete. This does not affect USB radios
because they poll the registers instead of using the IRQ line.

Signed-off-by: Douglas Fischer <fischerdouglasc@gmail.com>
[hans.verkuil@cisco.com: fixed 80 column checkpatch warning]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>