net: bonding: remove redudant brackets

It is not necessary to use two brackets. As such, the redudant brackets
are removed.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>