Updated TTY MAINTAINERS info

Greg Kroah-Hartman is the current TTY maintainer, however he wouldn't appear
listed as such upon running get_maintainers.pl for drivers under
drivers/tty/serial.

Signed-off-by: Lucas Kannebley Tavares <lucaskt@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>