bonding: remove unnecessary setup_by_slave member

We have a member called setup_by_slave in struct bonding to denote if the
bond dev has different type than ARPHRD_ETHER, but that is already denoted
in bond's netdev type variable if it was setup by the slave, so use that
instead of the member.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>