devlink: Return right error code in case of errors for region read

devlink_nl_cmd_region_read_dumpit() misses to return right error code on
most error conditions.
Return the right error code on such errors.

Fixes: 4e54795a

 ("devlink: Add support for region snapshot read command")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>