[media] af9035: fix warning

On a 32 bit system:

af9035.c: In function 'af9035_download_firmware':
af9035.c:446:3: warning: format '%lu' expects argument of type 'long unsigned
int', but argument 3 has type 'unsigned int' [-Wformat]

%zu avoids any warning on both 32 and 64 bit systems.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>