wl12xx: add system_hlid

system_hlid is a const hlid (always 0), used by the fw and driver
for packets which are not bound to specific role (e.g. dynamic
memory packets).
indicate it as always allocated.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>