Bluetooth: hci_uart: Remove unneeded cast of tty->disc_data

Casting the tty->disc_data to (void *) is not needed at all. So just
remove this cast and assign the object without a cast.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>