igb: Change IGB_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 igb devices can use as low as 64 descriptors.
This change will unify igb with other drivers.
Based on commit 7b1be198 ("e1000e: lower ring minimum size to 64")

Fixes: 9d5c8243 ("igb: PCI-Express 82575 Gigabit Ethernet driver")
Signed-off-by: Olga Zaborska <olga.zaborska@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>