cxgb4 : Update ipv6 address handling api

This patch improves on previously added support for ipv6 addresses. The code
is consolidated to a single file and adds an api for use by dependent upper
level drivers such as cxgb4i/iw_cxgb4 etc.

Signed-off-by: Anish Bhatt <anish@chelsio.com>
Signed-off-by: Deepak Singh <deepak.s@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>