STM32: remove F3 spi_api.c warning

Solve below warning:

"...\targets\TARGET_STM\TARGET_STM32F3\spi_api.c", line 73:
Warning:  #111-D: statement is unreachable

Author: LMESTM

targets/TARGET_STM/TARGET_STM32F3/spi_api.c

@@ -62,16 +62,22 @@ int spi_get_clock_freq(spi_t *obj) {
 #endif
 #if defined SPI2_BASE
 		case SPI_2:
+            /* SPI_2 and SPI_3. Source CLK is PCKL1 */
+			spi_hz = HAL_RCC_GetPCLK1Freq();
+			break;
 #endif
 #if defined SPI3_BASE
 		case SPI_3:
+			/* SPI_2 and SPI_3. Source CLK is PCKL1 */
+			spi_hz = HAL_RCC_GetPCLK1Freq();
+			break;
 #endif
 #if defined SPI4_BASE
 		case SPI_4:
-#endif
-			/* SPI_2 and SPI_3. Source CLK is PCKL1 */
-			spi_hz = HAL_RCC_GetPCLK1Freq();
+			/* SPI_4. Source CLK is PCKL2 */
+			spi_hz = HAL_RCC_GetPCLK2Freq();
 			break;
+#endif
 		default:
 			error("CLK: SPI instance not set");
             break;