[media] si2168: do not set values which are already on default

No need to set explicitly value that are already defaulted same.

Setting new value returns old value. Firmware default values can
be found just looking returned value.

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