Bluetooth: hci_bcm: Handle empty packet after firmware loading

The Broadcom controller on the Raspberry Pi3 sends an empty packet with
packet type 0x00 after launching the firmware. This will cause logging
of errors.

  Bluetooth: hci0: Frame reassembly failed (-84)

Since this seems to be an intented behaviour of the controller, handle
it gracefully by parsing that empty packet with packet type 0x00 and
then just simply report it as diagnostic packet.

With that change no errors are logging and the packet itself is actually
recorded in the Bluetooth monitor traces.

  < HCI Command: Broadcom Launch RAM (0x3f|0x004e) plen 4
         Address: 0xffffffff
  > HCI Event: Command Complete (0x0e) plen 4
       Broadcom Launch RAM (0x3f|0x004e) ncmd 1
         Status: Success (0x00)
  = Vendor Diagnostic (len 0)
  < HCI Command: Broadcom Update UART Baud Rate (0x3f|0x0018) plen 6
         00 00 00 10 0e 00                                ......
  > HCI Event: Command Complete (0x0e) plen 4
       Broadcom Update UART Baud Rate (0x3f|0x0018) ncmd 1
         Status: Success (0x00)
  < HCI Command: Reset (0x03|0x0003) plen 0
  > HCI Event: Command Complete (0x0e) plen 4
       Reset (0x03|0x0003) ncmd 1
         Status: Success (0x00)

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