Bluetooth: Fix issue with uninitialized nsh.type in DTL-1 driver

The current build shows a warning with the DTL-1 driver:

  CC [M]  drivers/bluetooth/dtl1_cs.o
drivers/bluetooth/dtl1_cs.c: In function ‘dtl1_hci_send_frame’:
drivers/bluetooth/dtl1_cs.c:396: warning: ‘nsh.type’ may be used uninitialized in this function

Fix this by adding a proper error for unknown packet types.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>