scsi: storvsc: Increase the ring buffer size

Increase the default ring buffer size as this can significantly
improve performance especially on high latency storage back-ends.

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