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

Fixes: d4e0fe01

 ("igbvf: add new driver to support 82576 virtual functions")
Signed-off-by: Olga Zaborska <olga.zaborska@intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>