tty: undo export of tty_open_by_driver

Since we have tty_kopen, we no longer need to export tty_open_by_driver.
This patch makes this function static.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>