drivers: eth: e1000: Enable multicast

Add "Multicast Promiscuous Enabled" (RCTL_MPE) bit definition and
use it for the receive control register (RCTL) initialization.

Multicast needs to be enabled in order for IPv6 auto-configuration
to succeed.

Signed-off-by: Oleg Zhurakivskyy <oleg.zhurakivskyy@intel.com>