[SCSI] aic79xx: use tcq functions

This patch converts aic79xx to use the midlayer-supplied tcq
functions.

We also set the queuedepth to '1' if tcq is disabled; the
aic79xx driver gets confused otherwise. Will set it back to
'2' once I figure out how to queue requests in the driver.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>