misc: make types consistent for agent_count variable

The variable agent_count for several protocols was
inconsistent in type, in some modules being signed and
in some modules being unsigned.

Make all agent_count variables of unsigned type.

Change-Id: I02957315f9a5048ab12c0f5d848dd0e402c3382a
Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>