MAINTAINERS: Update email address for Martyn Welch

I have recently left GE and the email address listed for me in the
maintainers file is no longer valid. Updating email address.

Signed-off-by: Martyn Welch <martyn@welchs.me.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>