[SCSI] advansys: Support 16-byte commands properly

The SCSI midlayer won't send commands greater than ->max_cmd_len.
So the checks on length in asc_build_req and adv_build_req are obsolete
and can be deleted, but also we have to set the max_cmd_len in
advansys_board_found().  Also move the length definitions together, and
write a helpful comment.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>