ixgbe: fix sfp_timer clean up in ixgbe_down

We weren't stoping the sfp_timer after the device was brought down.
This patch properly cleans up.

Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>