MAINTAINERS: Stephen Hemminger email change

I changed my email because the vyatta.com mail server is now
redirected to brocade.com; and the Brocade mail system
is not friendly to Linux desktop users.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>