[SCSI] ipr: implement shutdown changes and remove obsolete write cache parameter

This patch adds a reboot notifier that will issue a shutdown prepare command
to all adapters.  This helps to prevent a problem where the primary adapter can
get shut down before the secondary adapter and cause the secondary adapter to
fail over and log and error.
This patch also removes the "enable_cache" paramater as it is obsolete.  Write
cache for an adapter is now controlled from the iprconfig utility.

Signed-off-by: Wayne Boyer <wayneb@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>