bonding: improve nl error msg when device can't be enslaved because of IFF_MASTER

Use a more user friendly netlink error message when a device can't be
enslaved because it has IFF_MASTER, by not referring directly to a
kernel internal flag.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>