bdk: sdmmc: homogenize return values

The actual target of this refactoring.

bdk/storage/emmc.h

@@ -62,7 +62,7 @@ void emmc_error_count_increment(u8 type);
 u16 *emmc_get_error_count();
 u32  emmc_get_mode();
 int  emmc_init_retry(bool power_cycle);
-bool emmc_initialize(bool power_cycle);
+int  emmc_initialize(bool power_cycle);
 int  emmc_set_partition(u32 partition);
 void emmc_end();