scsi: storvsc: Always send on the selected outgoing channel

The current code always sent packets without data on the primary channel.
Properly distribute sending of packets with no data amongst all available
channels. I would like to thank Long Li for noticing this problem.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>