tpm.c: Don't try to put -1 in a variable of type TpmModel

The TpmModel type is an enum (valid values 0 and 1), which means
the compiler can legitimately decide that comparisons like
'tpm_models[i] == -1' are never true. (For example it could
pick 'unsigned char' as its type for representing the enum.)

Avoid this issue by using TPM_MODEL_MAX to mark entries in
the tpm_models[] array which aren't filled in, instead of -1.

This silences a clang warning:

 tpm.c:43:27: error: comparison of constant -1 with expression of type
      'enum TpmModel' is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (tpm_models[i] == -1) {
            ~~~~~~~~~~~~~ ^  ~~

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1375096931-13842-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
