bonding: permit enslaving interfaces without set_mac support

Currently we exit if the slave isn't the first slave, doesn't support mac
address setting and fail_over_mac isn't FOM_ACTIVE. It's wrong because we
only require ndo_set_mac_address in case bonding is in active-backup mode
and FOM isn't FOM_ACTIVE.

To fix this - only exit with an error if we're in a/b mode and have
fail_over_mac != FOM_ACTIVE.

Also, maintain current behaviour on the first slave (forcibly change fom to
FOM_ACTIVE) to not break anyone's configuration.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>