[media] m88ds3103: refactor firmware download

* remove some unneeded variable initialization
* rename variables
* use min() macro to calc max i2c xfer len
* change bad firmware error code from EFAULT to EINVAL

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>