[media] cx24120: i2c-max-write-size is now configurable

Some i2c-hosts are quite limited regarding maximum
i2c-burst-write-sizes. This patch makes the previously
hardcoded field configurable by users of the driver.

Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>