[SCSI] advansys: Remove `waiting' queue

If the adapter is busy, return the request to the midlayer rather than
queueing it in the driver.  asc_execute_queue() and asc_dequeue() become
unused, and we don't need to print out stats on the waiting queue any more.
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>