USB: serial: drop obsolete open-race workaround

Commit a65a6f14 ("USB: serial: fix race between probe and open")
fixed a race between probe and open, which could lead to crashes when a
not yet fully initialised port was being opened.

This race was later incidentally closed by commit 7e73eca6 ("TTY:
move cdev_add to tty_register_device") which moved character-device
registration from tty_register_driver to tty_register_device, which
isn't called until the port has been fully set up.

Remove the now redundant workaround which had the negative side effect
of not allowing a port to be opened immediately after user space had
been notified of a new tty device.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>