[SCSI] sd: add support for runtime PM

This patch (as1399) adds runtime-PM support to the sd driver.  The
support is unsophisticated: If a SCSI disk device is mounted, or if
its device file is held open, then the device will not be
runtime-suspended; otherwise it will (provided userspace gives
permission by writing "auto" to the sysfs power/control attribute).

In order to make this work, a dev_set_drvdata() call had to be moved
from sd_probe_async() to sd_probe().  Also, a few lines of code were
changed to use a local variable instead of recalculating the address
of an embedded struct device.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>