scsi: switch ->bios_param() to passing gendisk

Instances are passed struct block_device *bdev argument; the only thing
it is used for (if it's used in the first place) is bdev->bd_disk.
Might as well pass that in the first place...

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>