Fix PSA_MAX_PERSISTENT_KEY_IDENTIFIER to mean what it says

PSA_MAX_PERSISTENT_KEY_IDENTIFIER was actually one plus the maximum
key identifier. Change it to be the maximum value, and change the code
that uses it accordingly.

There is no semantic change here (the maximum value hasn't changed).
This commit only makes the implementation clearer.

library/psa_crypto_slot_management.c

@@ -201,7 +201,7 @@ static int psa_is_key_id_valid( psa_key_id_t key_id )
         return( 0 );
     /* Reject high values because the file names are reserved for the
      * library's internal use. */
-    if( key_id >= PSA_MAX_PERSISTENT_KEY_IDENTIFIER )
+    if( key_id > PSA_MAX_PERSISTENT_KEY_IDENTIFIER )
         return( 0 );
     return( 1 );
 }

library/psa_crypto_storage.h

@@ -59,7 +59,7 @@ extern "C" {
  * This limitation will probably become moot when we implement client
  * separation for key storage.
  */
-#define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xffff0000
+#define PSA_MAX_PERSISTENT_KEY_IDENTIFIER 0xfffeffff
 
 /**
  * \brief Format key data and metadata and save to a location for given key