misc: mark write buffer const

The write operation in misc_ops already takes a "const void *" buffer,
but misc_write() takes a mutable "void *".  There's no reason for this,
so make misc_write() consistent with the standard write() prototype.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/misc/misc-uclass.c b/drivers/misc/misc-uclass.c
index cbfacc3..cfe9d56 100644
--- a/drivers/misc/misc-uclass.c
+++ b/drivers/misc/misc-uclass.c
@@ -26,7 +26,7 @@
 	return ops->read(dev, offset, buf, size);
 }
 
-int misc_write(struct udevice *dev, int offset, void *buf, int size)
+int misc_write(struct udevice *dev, int offset, const void *buf, int size)
 {
 	const struct misc_ops *ops = device_get_ops(dev);