[SCSI] zfcp: Fix oops during shutdown of offline device

With the change that the zfcp_adapter struct is only allocated when
the device is set online, the shutdown handler has to check for a
non-existing zfcp_adapter struct. On the other hand, this check is not
necessary in the offline callback, since an online device has the
zfcp_adapter allocated and we go through the offline callback before
removing the ccw device.

Reviewed-by: Felix Beck <felix.beck@de.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>