[SCSI] sym53c8xx: fix shost use-after-free and memory leak

This patch fixes two bugs:

1) rmmod sym53c8xx uses shost after freeing it with
   scsi_put_host(shost).

2) insmod sym53c8xx doesn't call scsi_put_host(shost) if
   scsi_add_host() fails, causing a memory leak on the error path.

Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>