igbvf: remove unneeded cast

The cast and comment are unnecessary in the current upstream kernel.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Tested-by: Garrett, RobertX E <robertx.e.garrett@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>