cxgbe(4): Use ifaddr_event_ext instead of ifaddr_event for CLIP management.

The _ext event notification includes the address being added/removed and
that gives the driver an easy way to ignore non-IPv6 addresses. Remove
'tom' from the handler's name while here, it was moved out of t4_tom a
long time ago.

MFC after: 1 week
Sponsored by: Chelsio Communications