hw/isa/i82378: Remove dead assignment

hw/isa/i82378: Remove dead assignment

Rename the unique variable assigned as 'pit' which better
represents what it holds, to fix a warning reported by the
Clang static code analyzer:

    CC      hw/isa/i82378.o
  hw/isa/i82378.c:108:5: warning: Value stored to 'isa' is never read
      isa = isa_create_simple(isabus, "i82374");
      ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422133152.16770-7-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>