nec plugin: Compare OFS datapath_id as hex int

Previously NEC plugin compares old and new datapath_ids as
a string and zero padding in hex notation is not taken into
account when compared. This causes unintended deletion and
recreation of a port on OpenFlow controller. This patch fixes
this issue by comparing datapath_ids as hex int.

Change-Id: I6aa0a041e98c9bc489af89bb642ec5f86eaecce5
Closes-Bug: 1278349