ixgbe: Simplify definitions for regidx and bit in set_vfta

This patch simplifies the logic for setting the VFTA register by removing
the number of conditional checks needed.  Instead we just use some boolean
logic to generate vfta_delta, and if that is set then we xor the vfta by
that value and write it back.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>