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.