x86: Fix UV BAU destination subnode id

The SGI UV Broadcast Assist Unit is used to send TLB shootdown
messages to remote nodes of the system.  The header of the
message must contain the subnode id of the block in the
receiving hub that handles such messages.  It should always be
0x10, the id of the "LB" block.

It had previously been documented as a "must be zero" field.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Acked-by: Jack Steiner <steiner@sgi.com>
LKML-Reference: <E1Mc1x7-0005Ce-6t@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>