[media] si2157: clean logging

[media] si2157: clean logging

Same thing for si2157 as Antti did earlier for tda18212:

There is no need to print module name nor function name as those
are done by kernel logging system when dev_xxx logging is used and
driver is proper I2C driver.

While here, fix a typo ("unknown") in si2157_init.

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