target: Correct a comment

Correct the comment above the definition of TCM_MAX_COMMAND_SIZE.
A quote from Christoph:

    There aren't any legacy issues, we just decided to handle >
    16 byte CDBs in the slow path.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>
