[media] af9015: Fix max I2C message size when used with tda18271

Changeset 1724c8fa added an option to change
the maximum I2C size to 8 bytes. However, it forgot to replace the previous
usage at af9015 to use the newly defined macro value
(TDA18271_16_BYTE_CHUNK_INIT).

A latter changeset (e350d44f) extended the
possible values for .small_i2c field and, instead of using a random sequence
of numbers, it used a number that makes more sense (e. g. the actual limit,
in terms of bytes).

However, as af9015 were using .small_i2c = 1, this become undefined, and the
restriction of a max size of 16 was gone.

While here, fix the reported msg size at tda18271-common.c.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Tested-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>