i40evf: Use is_multicast_ether_addr helper

Use the is_multicast_ether_addr helper function from linux/etherdevice.h
instead of open coding the multicast address check.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>