ixgbe: define IXGBE_MAX_VFS_DRV_LIMIT macro and cleanup const 63

Because ixgbe driver limit the max number of VF
 functions could be enabled to 63, so define one macro IXGBE_MAX_VFS_DRV_LIMIT
 and cleanup the const 63 in code.

v3: revised for net-next tree.

Signed-off-by: Ethan Zhao <ethan.kernel@gmail.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>