sim: ppc: fix some Wenum-compare warnings

sim: ppc: fix some Wenum-compare warnings

When compiling with --enable-werror and CFLAGS="-O0 -g -Wall", we run into:
...
src/sim/ppc/hw_phb.c: In function 'hw_phb_attach_address':
src/sim/ppc/hw_phb.c:315:12: error: comparison between \
  'attach_type {aka enum _attach_type}' and \
  'enum <anonymous>' [-Werror=enum-compare]
   if (type != hw_phb_normal_decode
            ^~
...

Fix this by casting type to hw_phb_decode.