[SCSI] pm8001: potential null dereference in pm8001_dev_gone_notify()

In the original code we dereferenced "pm8001_dev" before checking if it
was null.  This patch moves the dereference inside the condition.

This was found by a static checker (smatch).  I looked, but I couldn't
tell if "pm8001_dev" dev was ever actually null.  The approach in this
patch seemed like the safest response.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>