mtd: return error code from mtd_unpoint

The 'mtd_unpoint()' API function should be able to return an error code because
it may fail if you specify incorrect offset. This patch changes this MTD API
function and amends all the drivers correspondingly.

Also return '-EOPNOTSUPP' from 'mtd_unpoint()' when the '->unpoint()' method is
undefined. We do not really need this currently, but this just makes
sense to be consistent with 'mtd_point()'.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/devices/mtdram.c

@@ -60,8 +60,9 @@ static int ram_point(struct mtd_info *mtd, loff_t from, size_t len,
 	return 0;
 }
 
-static void ram_unpoint(struct mtd_info *mtd, loff_t from, size_t len)
+static int ram_unpoint(struct mtd_info *mtd, loff_t from, size_t len)
 {
+	return 0;
 }
 
 /*