[media] tuner-i2c: be consistent with I2C declaration

On alpha, gcc warns a log about signed/unsigned ballance, with
produces 3185 warnings. Ok, this is bogus, but it indicates that
the declaration at V4L2 side is not consistent with the one at
I2C.

With this trivial patch, the number of errors reduce to 2959
warnings. Still too much, but it is 7.1% less. So let's do it.

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