mmc: core: Convert simple_stroul to kstroul

simple_strtoul() is obsolete and lacks proper error handling, making it
unsafe for converting strings to unsigned long values. Replace it with
kstrtoul(), which provides robust error checking and better safety.

This change improves the reliability of the string-to-integer conversion
and aligns with current kernel coding standards. Error handling is added
to catch conversion failures, returning -EINVAL when input is invalid.

Issue reported by checkpatch:
- WARNING: simple_strtoul is obsolete, use kstrtoul instead

Signed-off-by: Riyan Dhiman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/core/block.c

···
 			      const char *buf, size_t count)
 {
 	int ret;
-	char *end;
 	struct mmc_blk_data *md = mmc_blk_get(dev_to_disk(dev));
-	unsigned long set = simple_strtoul(buf, &end, 0);
-	if (end == buf) {
+	unsigned long set;
+
+	if (kstrtoul(buf, 0, &set)) {
 		ret = -EINVAL;
 		goto out;
 	}