[SCSI] Fix module removal/device add race

This patch (as546) fixes an oops-causing failure to check the return code
from scsi_device_get.  The call can return an error if the LLD is being
unloaded from memory.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>