Bluetooth: hci_uart: Fix zero len data packet reception issue

Packets with a variable length value equal to zero were not received.

Since no more data expected (and input buffer entirely consumed), we
need to complete/forward the packet immediately instead of waiting for
more data.

Signed-off-by: Loic Poulain <loic.poulain@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>