sd: Disable support for 256 byte/sector disks

256 bytes per sector support has been broken since 2.6.X,
and no-one stepped up to fix this.
So disable support for it.

Signed-off-by: Mark Hounschell <dmarkh@cfl.rr.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Odin.com>