tty/serial/mux.c: linux/tty.h included twice

drivers/tty/serial/mux.c included 'linux/tty.h' twice, remove
the duplicate.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>