eventdev: set error code in port link/unlink functions

eventdev: set error code in port link/unlink functions

The return value for rte_event_port_{link, unlink}() is defined as the
"number of {links, unlinks} actually established." However, the eventdev
layer's error checking returns negative error values. This commit aligns
the eventdev code with the API definition by having it set rte_errno and
return 0 if it detects an error.

Fixes: 4f0804bbdfb9 ("eventdev: implement the northbound APIs")
Cc: stable@dpdk.org
Signed-off-by: Gage Eads <gage.eads@intel.com>