scsi: do not report bogus overruns for commands in the 0x00-0x1F range

Interpreting cdb[4] == 0 as a request to transfer 256 blocks is only
needed for READ_6 and WRITE_6.  No other command in that range needs
that special-casing, and the resulting overrun breaks scsi-testsuite's
attempt to use command 2 as a known-invalid command.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
