fix(xiao): name SPI3 MOSI alternate function

Author: cumin777

variants/STM32C5xx/C593C(E-G)(T-U)_C5A3CG(T-U)/variant_XIAO_STM32C5.h

@@ -32,6 +32,7 @@
 /* Alternate functions used by the default board peripherals. */
 #define PA9_ALT1                (PA9 | ALT1)
 #define PA10_ALT1               (PA10 | ALT1)
+#define PB15_ALT2               (PB15 | ALT2)
 
 #define NUM_DIGITAL_PINS        21
 #define NUM_ANALOG_INPUTS       4
@@ -46,7 +47,7 @@
 #define PIN_SPI_SS1             NUM_DIGITAL_PINS
 #define PIN_SPI_SS2             NUM_DIGITAL_PINS
 #define PIN_SPI_SS3             NUM_DIGITAL_PINS
-#define PIN_SPI_MOSI            (PB15 | ALT2)
+#define PIN_SPI_MOSI            PB15_ALT2
 #define PIN_SPI_MISO            PB0
 #define PIN_SPI_SCK             PE2