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 <[email protected]>
Message-id: [email protected]
Signed-off-by: Anthony Liguori <[email protected]>

tpm.c

@@ -32,15 +32,15 @@ static TPMDriverOps const *be_drivers[TPM_MAX_DRIVERS] = {
 };
 
 static enum TpmModel tpm_models[TPM_MAX_MODELS] = {
-    -1,
+    TPM_MODEL_MAX,
 };
 
 int tpm_register_model(enum TpmModel model)
 {
     int i;
 
     for (i = 0; i < TPM_MAX_MODELS; i++) {
-        if (tpm_models[i] == -1) {
+        if (tpm_models[i] == TPM_MODEL_MAX) {
             tpm_models[i] = model;
             return 0;
         }