ixgbe: move num_vfs_macvlans allocation into separate function

Move the code allocating memory for list of MAC addresses that
the VFs can use for MACVLAN into its own function.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>