Propagate SWITCH_RATIO to DYNAMIC_ARCH builds

Previously dynamic builds were either using the default SWITCH_RATIO
or one from the higher level architecture; this patch ensures the
dynamic builds can use this parameter as well.

lapack/potrf/potrf_parallel.c

@@ -80,10 +80,6 @@ static FLOAT dm1 = -1.;
 #define DIVIDE_RATE 2
 #endif
 
-#ifndef SWITCH_RATIO
-#define SWITCH_RATIO 2
-#endif
-
 #ifndef LOWER
 #define TRANS
 #endif