MAINTAINERS: update Corey Thomas email address

On Sat, 2007-12-22 at 18:08 -0500, Corey Thomas wrote:
> This email, coreythomas@charter.net is good.  ISP change.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>