Drivers: hv: vmbus: Set ring_info field to 0 and remove memset

Set "ring_info->priv_read_index" to 0. Now, all of ring_info's fields
are explicitly set in this function. The memset() call is no longer
necessary, so remove it.

Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>