[dwm] bad operator precedence in aspect revert

Hi Anselm,

Changeset 1343:f4707b7bd3a2 introduced erroneous parentheses in
(float)(w / h) and (float)(h / w). Now there is an integer devision,
and the result is converted to float, instead of converting the
denominator to force a float division.

This leads to funky behavior when resizing an mplayer window. A fix
is attached.

Regards,
Peter