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>

drivers/net/ethernet/intel/igb/e1000_defines.h

@@ -301,6 +301,8 @@
 							* transactions */
 #define E1000_DMACR_DMAC_LX_SHIFT       28
 #define E1000_DMACR_DMAC_EN             0x80000000 /* Enable DMA Coalescing */
+/* DMA Coalescing BMC-to-OS Watchdog Enable */
+#define E1000_DMACR_DC_BMC2OSW_EN	0x00008000
 
 #define E1000_DMCTXTH_DMCTTHR_MASK      0x00000FFF /* DMA Coalescing Transmit
 							* Threshold */

drivers/net/ethernet/intel/igb/igb_main.c

@@ -7147,6 +7147,9 @@ static void igb_init_dmac(struct igb_adapter *adapter, u32 pba)
 
 			/* watchdog timer= +-1000 usec in 32usec intervals */
 			reg |= (1000 >> 5);
+
+			/* Disable BMC-to-OS Watchdog Enable */
+			reg &= ~E1000_DMACR_DC_BMC2OSW_EN;
 			wr32(E1000_DMACR, reg);
 
 			/*