[media] uapi/media.h: Use u32 for the number of graph objects

While we need to keep a u64 alignment to avoid compat32 issues,
having the number of entities/pads/links/interfaces represented
by an u64 is incoherent with the ID number, with is an u32.

In order to make it coherent, change those quantities to u32.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>