[media] si2157: Use name si2157_ops instead of si2157_tuner_ops

The struct prototype is defined at the beginning of the code as
"si2157_ops" but the real struct is called "si2157_tuner_ops".

This is causing the name to be empty on this info msg: si2157 16-0060:
si2157: found a '' in cold state

[crope@iki.fi: commit msg from Luis email reply]
Signed-off-by: Olli Salonen <olli.salonen@iki.fi>
Cc: Luis Alves <ljalvs@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>