[media] fc2580: cleanups and variable renames

Rename driver state from priv to dev.
Remove legacy i2c-gate control.
Use I2C client for proper dev_() logging.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>