bonding: bond_open error return value

The convention for API functions in kernel is to return errno value;
bond_open would return -1 if alb setup failed. The only reason that
could happen is if kmalloc() failed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>