Rename PSA_ALG_STREAM_CIPHER -> PSA_ALG_STREAM_CIPHER_BASE

Follow the usual naming convention: PSA_ALG_xxx_BASE for a constant
that isn't an algorithm, just used to build one.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 9145a6d..50a9990 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1191,7 +1191,7 @@
 
         switch( alg )
         {
-            case PSA_ALG_STREAM_CIPHER:
+            case PSA_ALG_STREAM_CIPHER_BASE:
                 mode = MBEDTLS_MODE_STREAM;
                 break;
             case PSA_ALG_CBC_BASE: