scsi: put runtime PM reference for well-known logical units

Some well-known logical units might not have scsi upper-layer
drivers. In such cases, the runtime PM reference count increased
during device enumeration will not be decreased, causing the
parent device (host) to always be on even during idle.

Change-Id: I458007425231e5e245cd39494c9fb42a8d4cb9cd
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>