TTY: hci_ldisc, remove invalid check in open

hci_ldisc's open checks if tty_struct->disc_data is set. And if so it
returns with an error. But nothing ensures disc_data to be NULL. And
since ld->ops->open shall be called only once, we do not need the
check at all. So remove it.

Note that this is not an issue now, but n_tty will start using the
disc_data pointer and this invalid 'if' would trigger then rendering
TTYs over BT unusable.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Johan Hedberg <johan.hedberg@gmail.com>
Cc: linux-bluetooth@vger.kernel.org
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>