ACL: Allow short packets

ACL packets always contain L2CAP packets, so the assembler
uses the size of the L2CAP packet to determine when a
packet is completed.  If packets are smaller than the size
of an L2CAP header, wait to see if more will come before
discarding them.

Bug: 292933138
Test: atest bluetooth_test_gd_unit
Change-Id: I010f9c8b31eb4d2298cc53c9ff0a9bd6b180dc01