[SCSI] aic79xx: remove scsi_assign_lock usage

just take the internal lock in queuecommand instead.  also switch
the only direct use of the internal lock to the wrappers used elsewhere.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>