samd/pin_af: Fix a typo in a conditional compile.

Thanks to @ricksorensen for finding it.
Signed-off-by: robert-hh <robert@hammelrath.com>

ports/samd/pin_af.c

@@ -112,7 +112,7 @@ adc_config_t get_adc_config(int pin_id, int32_t flag) {
     const machine_pin_obj_t *pct_ptr = pin_find_by_id(pin_id);
     if (pct_ptr->adc0 != 0xff && (flag & (1 << pct_ptr->adc0)) == 0) {
         return (adc_config_t) {0, pct_ptr->adc0};
-    #if defined(MUC_SAMD51)
+    #if defined(MCU_SAMD51)
     } else if (pct_ptr->adc1 != 0xff && (flag & (1 << (pct_ptr->adc1 + 16))) == 0) {
         return (adc_config_t) {1, pct_ptr->adc1};
     #endif