[media] tda18218: fix compile warning

drivers/media/common/tuners/tda18218.c: In function 'tda18218_wr_regs':
drivers/media/common/tuners/tda18218.c:58:5: warning: 'ret' may be used uninitialized in this function

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>