fix numbering for soft-SPI (tested)

Author: ladyada

variants/feather_m4/variant.h

@@ -137,8 +137,8 @@ static const uint8_t ATN = PIN_ATN;
 #define SPI_INTERFACES_COUNT 1
 
 #define PIN_SPI_MISO         (28u)
-#define PIN_SPI_MOSI         (30u)
-#define PIN_SPI_SCK          (29u)
+#define PIN_SPI_MOSI         (29u)
+#define PIN_SPI_SCK          (30u)
 #define PERIPH_SPI           sercom4
 #define PAD_SPI_TX           SPI_PAD_0_SCK_1
 #define PAD_SPI_RX           SERCOM_RX_PAD_3