mtd: nand: sunxi: fix potential divide-by-zero error

clk_round_rate() can return <= 0. Currently the value returned by
clk_round_rate() is used directly for a division. This patch introduces a
guard to ensure a divide-by-zero or a divide by a negative number for that
matter can't happen by bugging out returning -EINVAL if clk_round_rate()
returns <= 0.

Fixes: 2d43457f ("mtd: nand: sunxi: fix EDO mode selection")
Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/sunxi_nand.c

@@ -1728,6 +1728,10 @@ static int sunxi_nfc_setup_data_interface(struct mtd_info *mtd, int csline,
 	 */
 	chip->clk_rate = NSEC_PER_SEC / min_clk_period;
 	real_clk_rate = clk_round_rate(nfc->mod_clk, chip->clk_rate);
+	if (real_clk_rate <= 0) {
+		dev_err(nfc->dev, "Unable to round clk %lu\n", chip->clk_rate);
+		return -EINVAL;
+	}
 
 	/*
 	 * ONFI specification 3.1, paragraph 4.15.2 dictates that EDO data