[SCSI] hpsa: do not attempt to flush the cache on locked up controllers

There's no point in trying since it can't work, and if you do
try, it will just hang the system on shutdown.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>