scsi: return blk_status_t from device handler ->prep_fn

Remove the last use of the old BLKPREP_* values, which get converted
to BLK_STS_* later anyway.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>