opt_dff: fix sequence point copy paste bug

opt_dff: fix sequence point copy paste bug

Newer GCCs emit the following warning for opt_dff:

    passes/opt/opt_dff.cc:560:17: warning: operation on ‘ff.Yosys::FfData::has_clk’ may be undefined [-Wsequence-point]
      560 |      ff.has_clk = ff.has_ce = ff.has_clk = false;
          |      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Which is correct: the order of whether the read or write of has_clk
occurs first is undefined since there is no sequence point between them.

This is almost certainly just a typo/copy paste error and objectively
wrong, so just fix it.

Signed-off-by: Austin Seipp <aseipp@pobox.com>