of: overlay: pr_err from return NOTIFY_OK to overlay apply/remove

A device tree overlay notifier can return NOTIFY_OK, NOTIFY_STOP,
or an embedded errno.  overlay_notify() incorrectly reports an
error for NOTIFY_OK.

Reported-by:  <atull@kernel.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>