scsi: core: Clean up SG_NONE

Remove SG_NONE and a related misleading comment. Update documentation.

This patch does not affect behaviour as zero initialization is redundant.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Viresh Kumar <vireshk@kernel.org>
Cc: Oliver Neukum <oneukum@suse.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: usb-storage@lists.one-eyed-alien.net
Link: https://lore.kernel.org/r/b4779b7a6563f6bd8d259ee457871c1c463c420e.1572656814.git.fthain@telegraphics.com.au


Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>