[media] si2168: Implement own I2C adapter locking

We need own I2C locking because of tuner I2C adapter/repeater.
Firmware command is executed using I2C send + reply message. Default
I2C adapter locking protects only single I2C operation, not whole
send + reply sequence as needed. Due to that, it was possible tuner
I2C message interrupts firmware command sequence.

Reported-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>