[media] af9013: reimplement firmware download

Split FW download packages smarter way and bug free. Implementation is
based of Andrea Merello's example he provided for tda18218 driver.
Count remaining FW bytes down in loop instead of division and modulo
combination used earlier.

Thanks to: Andrea Merello <andrea.merello@gmail.com>

Signed-off-by: Antti Palosaari <crope@iki.fi>
Cc: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>