Bluetooth: Fix possible NULL dereference in hci_uart_tty_receive

This patch adds a NULL check for the HCI UART ldisc driver because some
of HCI UART drivers allow hci_uart_tty_receive function to be called
even though the HCI device hasn't been registered yet.

Signed-off-by: Chan-yeol Park <chanyeol.park@samsung.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>