Limit fuzzed L2CAP packets to 64K

The maximum ACL packet size is 64K, so fuzzing beyond that for L2CAP
is not worthwhile.

Tested with packets smaller than the limit to verify that the
failure only occurs with packets beyond the limit.

Bug: 146869530
Test: bluetooth_gd_fuzz_test \
      clusterfuzz-testcase-minimized-bluetooth_gd_fuzz_test-5672164365697024
Change-Id: I1361fb4904d6c1a55caad63bf65c67bef0207362