e1000: unset IFF_UNICAST_FLT on WMware 82545EM

VMWare's e1000 implementation does not seem to support unicast filtering.
This can be observed by configuring a macvlan interface on eth0 in a VM in
VMWare Fusion 5.0.5, and trying to use that interface instead of eth0.
Tested on 3.16.

Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>