Probing clock frequency should be 400kHz, not 4MHz.
int max_freq_khz; /*!< max frequency supported by the host */
#define SDMMC_FREQ_DEFAULT 20000 /*!< SD/MMC Default speed (limited by clock divider) */
#define SDMMC_FREQ_HIGHSPEED 40000 /*!< SD High speed (limited by clock divider) */
-#define SDMMC_FREQ_PROBING 4000 /*!< SD/MMC probing speed */
+#define SDMMC_FREQ_PROBING 400 /*!< SD/MMC probing speed */
float io_voltage; /*!< I/O voltage used by the controller (voltage switching is not supported) */
esp_err_t (*init)(void); /*!< Host function to initialize the driver */
esp_err_t (*set_bus_width)(int slot, size_t width); /*!< host function to set bus width */