mtd: remove R/O checking duplication

Many drivers check whether the partition is R/O and return -EROFS if yes.
Let's stop having duplicated checks and move them to the API functions
instead.

And again a bit of noise - deleted few too sparse newlines, sorry.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index 317a80c..fa20a8f 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -268,6 +268,8 @@
 	ops->retlen = ops->oobretlen = 0;
 	if (!mtd->_write_oob)
 		return -EOPNOTSUPP;
+	if (!(mtd->flags & MTD_WRITEABLE))
+		return -EROFS;
 	return mtd->_write_oob(mtd, to, ops);
 }