[efi] Reset multicast filter list when setting SNP receive filters

According to the UEFI specification, the MCastFilter parameter (which
we currently pass as NULL, along with a zero MCastFilterCnt) is
optional only if ResetMCastFilter is true.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/efi/snpnet.c

 		    EFI_SIMPLE_NETWORK_RECEIVE_BROADCAST |
 		    EFI_SIMPLE_NETWORK_RECEIVE_PROMISCUOUS |
 		    EFI_SIMPLE_NETWORK_RECEIVE_PROMISCUOUS_MULTICAST );
-	if ( ( efirc = snp->snp->ReceiveFilters ( snp->snp, filters, 0, FALSE,
+	if ( ( efirc = snp->snp->ReceiveFilters ( snp->snp, filters, 0, TRUE,
 						  0, NULL ) ) != 0 ) {
 		rc = -EEFI ( efirc );
 		DBGC ( snp, "SNP %s could not set receive filters: %s\n",