spi.c fix output of warning for wrong baudrate

* do crude cpp-calculation for case when we want to warn

firmware/shared/spi.c

@@ -45,9 +45,18 @@ void spi_init(){
 		return d-1;  // the -1 because minimum divisor is /2
 	}
 
+	/* do pseudo-calculation in preprocessor, to get warnings 
+	 * we right-shift 8 times; if result >0, d will be >7
+	 * so output warning in that case
+	 */
+#define D_VAL (F_CPU / SPI_BAUDRATE)
+#if ((D_VAL >> 8) > 0 )
+#warning  "spi baudrate too slow, cannot be set"
+#endif
+
 	d = spi_clock_divisor();
 	if(d>7){
-                #warning "spi baudrate too slow, cannot be set"
+		// warning was (hopefully) output previously by cpp
 		d=7;
 	}