bonding: fix a parameter name in error message

When parsing module parameters, bond_check_params() erroneously use
'xor_mode' as the name of a module parameter in an error message.

The right name for this parameter is 'xmit_hash_policy'.

Signed-off-by: Nicolas de Pesloüan <nicolas.2p.debian@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>