mac_scsi: Add module option to Kconfig

Allow mac_scsi to be built as a module. Replace the old validation of
__setup options with code that validates both module and __setup options.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>