[media] media: radio-cadet, initialize timer with setup_timer

Stop accessing timer struct members directly and use the setup_timer
helper intended for that use. It makes the code cleaner and will allow
for easier change of the timer struct internals.

Signed-off-by: Matej Hulín <mito.hulin@gmail.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>