phosphor-state-manager: use CHASSIS_INSTANCE for chassis service

Some systems have more chassis than hosts.  Use the
OBMC_CHASSIS_INSTANCES variable instead of OBMC_HOST_INSTANCES for
instantiating the service symlinks.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I42eb92a68023788fc8c06e63ed3eb6eb714bf55c