[MTD] CHIPS: Do not oops when configured flash size is less then chip size

My kernel oopsed when the on-board flash chips where quadruple the size of the
platform default physmap_map.size. Turned out that max_chips becomes zero,
resulting in an oops later in locking chip->mutex being non-initialized.

Full debug trace available.

Signed-off-by: Leon Woestenberg <leonw@mailcan.com>

diff --git a/drivers/mtd/chips/gen_probe.c b/drivers/mtd/chips/gen_probe.c
index 9b252d2..9b7f82b 100644
--- a/drivers/mtd/chips/gen_probe.c
+++ b/drivers/mtd/chips/gen_probe.c
@@ -100,6 +100,11 @@ #endif
 	 * Align bitmap storage size to full byte.
 	 */
 	max_chips = map->size >> cfi.chipshift;
+	if (max_chips == 0) {
+		printk(KERN_WARNING "Single flash chip size exceeds the configured flash area size. Check your kernel configuration.\n");
+		kfree(cfi.cfiq);
+		return NULL;
+	}
 	mapsize = (max_chips / 8) + ((max_chips % 8) ? 1 : 0);
 	chip_map = kmalloc(mapsize, GFP_KERNEL);
 	if (!chip_map) {