bonding:delete a dereference before check

Dan Carpenter found that there was a dereference before a check,
added in 56d00c67(bonding:delete lacp_fast from ad_bond_info).

Signed-off-by: Weiping Pan <panweiping3@gmail.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>