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>