tty: define tty_open_by_driver when CONFIG_TTY is not defined

This patch adds definition of tty_open_by_driver when CONFIG_TTY is not
defined. This was supposed to have been included in commit
12e84c71 ("tty: export
tty_open_by_driver"). The patch follows convention for other such
functions and returns NULL.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>