net: thunderx: rework xcast message structure to make it fit into 64 bit

To communicate to PF each of ThunderX NIC VF uses mailbox which is
pair of 64 bit registers available to both VFn and PF.

This commit is to change the xcast message structure in order to
fit it into 64 bit.

Signed-off-by: Vadim Lomovtsev <vlomovtsev@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>