scsi: storvsc: Size the queue depth based on the ringbuffer size

Size the queue depth based on the ringbuffer size. Also accommodate for the
fact that we could have multiple channels (ringbuffers) per adaptor.

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