bonding: fix miimon failure counter

During the rework of the mii monitor for:

  commit f0c76d61
  Author: Jay Vosburgh <fubar@us.ibm.com>
  Date:   Wed Jul 2 18:21:58 2008 -0700

    bonding: refactor mii monitor

I left out the increment of the link failure counter.  This
patch corrects that omission.

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>