igc: Add Receive Descriptor Minimum Threshold Count to clear HW counters

The statistics of this register are being tracked, however, the register
was inadvertently missed when implementing igc_clear_hw_cntrs_base().
The register is clear on read, so add it to the function so that the
register is cleared when requested so the tracked count is accurate.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igc/igc_mac.c

@@ -308,6 +308,7 @@ void igc_clear_hw_cntrs_base(struct igc_hw *hw)
 	rd32(IGC_TLPIC);
 	rd32(IGC_RLPIC);
 	rd32(IGC_HGPTC);
+	rd32(IGC_RXDMTC);
 	rd32(IGC_HGORCL);
 	rd32(IGC_HGORCH);
 	rd32(IGC_HGOTCL);