igb: Disable the BMC-to-OS Watchdog Enable bit for DMAC.

Under certain scenarios, it's possible that bursty manageability traffic
over the BMC-to-OS path may overrun the internal manageability receive
buffer causing dropped manageability packets. Clearing this bit prevents
this situation by interrupting coalescing to allow manageability traffic
through.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>