media: si2157: use a different namespace for firmware

Each chip at the si21xx TER family seems to have a different firmware,
with seems to actually be a patch against the ROM code.

Rework the code in order to use different firmware files depending
on the chip ID and rom ID.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>