SPI setup will not yield the exact selected speed.

third_party/mfrc522_integration/mfrc522_rp2040_integration.c

@@ -166,8 +166,12 @@ uint8_t mfrc522_interface_spi_init(void)
     gpio_set_function(MFRC522_CFG_SPI_CLK_GPIO, GPIO_FUNC_SPI);
     gpio_set_function(MFRC522_CFG_SPI_SS_GPIO, GPIO_FUNC_SPI);
 
-    uint baudrate = MFRC522_CFG_SPI_SPEED_MHZ;
-    return (baudrate == spi_init(spi0, baudrate)) ? 0 : 1;
+    /*
+     * From the manual:
+     * "There is no guarantee that the baudrate requested can be achieved exactly;
+     *    the nearest will be chosen and returned"
+     */
+    return !spi_init(spi0, MFRC522_CFG_SPI_SPEED_MHZ);
 }
 
 /**