SCSI: Fix error handling when no ULD is attached

SCSI: Fix error handling when no ULD is attached

Commit 18a4d0a22ed6 ("[SCSI] Handle disk devices which can not process
medium access commands") introduced a bug in which we would attempt to
dereference the scsi driver even when the device had no ULD attached.

Ensure that a driver is registered and make the driver accessor function
more resilient to errors during device discovery.

Reported-by: Elric Fu <[email protected]>
Reported-by: Bart Van Assche <[email protected]>
Signed-off-by: Martin K. Petersen <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>