eventdev: fix links map initialization for SW PMD

eventdev: fix links map initialization for SW PMD

This patch initializes the links_map array entries to
EVENT_QUEUE_SERVICE_PRIORITY_INVALID, as expected by
rte_event_port_links_get(). This is necessary for the sw eventdev PMD,
which does not initialize links_map when rte_event_port_setup() calls
rte_event_port_unlink().

Fixes: 4f0804bbdfb9 ("eventdev: implement the northbound APIs")
Signed-off-by: Gage Eads <gage.eads@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>