Made the 2nd speed lookup table condition more explicit.

Marlin/speed_lookuptable.h

 { 992, 4}, { 988, 4}, { 984, 4}, { 980, 4}, { 976, 4}, { 972, 4}, { 968, 3}, { 965, 3}
 };
 
-#else
+#elif F_CPU == 20000000
 
 const uint16_t speed_lookuptable_fast[256][2] PROGMEM = {
    {62500, 54055}, {8445, 3917}, {4528, 1434}, {3094, 745}, {2349, 456}, {1893, 307}, {1586, 222}, {1364, 167},