Fix "int vs enum" warnings from armcc v5

enumerated type mixed with another type
diff --git a/library/rsa.c b/library/rsa.c
index 5f86173..2338264 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -526,7 +526,7 @@
     if( f_rng == NULL )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
-    md_info = md_info_from_type( ctx->hash_id );
+    md_info = md_info_from_type( (md_type_t) ctx->hash_id );
     if( md_info == NULL )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
@@ -705,7 +705,7 @@
     if( ilen < 16 || ilen > sizeof( buf ) )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
-    md_info = md_info_from_type( ctx->hash_id );
+    md_info = md_info_from_type( (md_type_t) ctx->hash_id );
     if( md_info == NULL )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );
 
@@ -943,7 +943,7 @@
         hashlen = md_get_size( md_info );
     }
 
-    md_info = md_info_from_type( ctx->hash_id );
+    md_info = md_info_from_type( (md_type_t) ctx->hash_id );
     if( md_info == NULL )
         return( POLARSSL_ERR_RSA_BAD_INPUT_DATA );