net: gptp: Return proper port number for given interface

The port number is stored starting from 0 in Ethernet context.
But in gPTP, it is an index which starts from 1. So increase
the value by 1 for a value returned from Ethernet context.

Fixes #26080

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>