Add tls12_prf_read for the new API

Technically we could have reused the old one for the new API, but then
we had to set an extra field during setup. The new version works when
all the fields that haven't been set explicitely are zero-initialised.

include/psa/crypto_struct.h

@@ -250,8 +250,8 @@ typedef struct psa_tls12_prf_key_derivation_s
 #endif
 
     /* Indicates how many bytes in the current HMAC block have
-     * already been read by the user. */
-    uint8_t offset_in_block;
+     * not yet been read by the user. */
+    uint8_t left_in_block;
 
     /* The 1-based number of the block. */
     uint8_t block_number;