igc: Change IGC_MIN to allow set rx/tx value between 64 and 80

Change the minimum value of RX/TX descriptors to 64 to enable setting the rx/tx
value between 64 and 80. All igc devices can use as low as 64 descriptors.
This change will unify igc with other drivers.
Based on commit 7b1be198 ("e1000e: lower ring minimum size to 64")

Fixes: 0507ef8a ("igc: Add transmit and receive fastpath and interrupt handlers")
Signed-off-by: Olga Zaborska <olga.zaborska@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>