bonding: fix kstrtou8() return value verification in num_peer_notif

It returns 0 in case of success, !0 error otherwise. Fix the improper error
verification.

Fixes: 2c9839c1 ("bonding: add num_grat_arp attribute netlink support")
CC: sfeldma@cumulusnetworks.com
CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>