bonding: remove the return value for bond_3ad_bind_slave()

I'm sure the operand slave and bond for the function will not
be NULL, so the check for the bond will not make any sense, so
remove the judgement, and the return value was useless here,
remove the unwanted return value.

The comments for the bond 3ad is too old, cleanup some errors
and warming.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>