devlink: Unlock on error in dumpit()

This needs to unlock before returning.

Fixes: 544e7c33

 ("net: devlink: Add support for port regions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201026080127.GB1628785@mwanda


Signed-off-by: Jakub Kicinski <kuba@kernel.org>