Bluetooth: hci_uart: Use const data pointer for received data

The TTY layer provides its data pointers as const, but the HCI UART
callbacks expect them as general data pointers. This is of course
wrong and instead of casting them, just fix the individual drivers
to actually take a const data pointer.

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