MAINTAINERS: fix my email address

As I'm not working for Texas Instruments anymore,
balbi@ti.com isn't a valid address. I'll be using
balbi@kernel.org at least for the time being.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Felipe Balbi <balbi@kernel.org>