sf: Guard against zero erasesize

With tiny SPI flash the erasesize is 0 which can cause a divide-by-zero
error. Check for this and return a proper error instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/mtd/spi/sf_probe.c b/drivers/mtd/spi/sf_probe.c
index e192f97..de6516f 100644
--- a/drivers/mtd/spi/sf_probe.c
+++ b/drivers/mtd/spi/sf_probe.c
@@ -189,7 +189,8 @@
 	struct mtd_info *mtd = &flash->mtd;
 	struct erase_info instr;
 
-	if (offset % mtd->erasesize || len % mtd->erasesize) {
+	if (!mtd->erasesize ||
+	    (offset % mtd->erasesize || len % mtd->erasesize)) {
 		debug("SF: Erase offset/length not multiple of erase size\n");
 		return -EINVAL;
 	}