mtd: nand: use ssize_t instead of size_t to prevent infinite loop

When a all 0xFF buffer is passed to drop_ffs, the no-0xFF check loop
will loop forever.
After the fix, If ssize_t i = -1 and size_t l = i + 1, the value of l
will still be 0 as expected.

Signed-off-by: Tao Hou <hotforest@gmail.com>
Cc: Ben Gardiner <bengardiner@nanometrics.ca>
Cc: Scott Wood <scottwood@freescale.com>

drivers/mtd/nand/nand_util.c

@@ -458,7 +458,8 @@ static int check_skip_len(nand_info_t *nand, loff_t offset, size_t length,
 static size_t drop_ffs(const nand_info_t *nand, const u_char *buf,
 			const size_t *len)
 {
-	size_t i, l = *len;
+	size_t l = *len;
+	ssize_t i;
 
 	for (i = l - 1; i >= 0; i--)
 		if (buf[i] != 0xFF)