tpm: avoid clang shifting negative signed warning

clang 3.7.0 on x86_64 warns about the following:

  hw/tpm/tpm_tis.c:1000:36: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
            tis->loc[c].iface_id = TPM_TIS_IFACE_ID_SUPPORTED_FLAGS1_3;
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  hw/tpm/tpm_tis.c:144:10: note: expanded from macro 'TPM_TIS_IFACE_ID_SUPPORTED_FLAGS1_3'
     (~0 << 4)/* all of it is don't care */)
      ~~ ^

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
