fluke-dmm: Fix two compiler warnings.

fluke-dmm: Fix two compiler warnings.

  src/hardware/fluke-dmm/fluke.c:312:24: warning: implicit conversion from \
  enumeration type 'enum sr_unit' to different enumeration type 'enum sr_mq' \
   [-Wenum-conversion]
          devc->mq = devc->unit = devc->mqflags = 0;
                   ~ ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~

  src/hardware/fluke-dmm/fluke.c:312:40: warning: implicit conversion from \
  enumeration type 'enum sr_mqflag' to different enumeration type 'enum \
  sr_unit' [-Wenum-conversion]
          devc->mq = devc->unit = devc->mqflags = 0;
                                ~ ~~~~~~~~~~~~~~^~~