[media] si2168: enhance firmware download routine

All known old firmware firmware formats are downloaded using 8 byte
chunks. Reject firmware if it could not be divided to 8 byte chunks
and because of that we could simplify some calculations. Now both
supported firmware download routines are rather similar.

Cc: Olli Salonen <olli.salonen@iki.fi>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>