[SCSI] aic94xx: Scan SAS devices asynchronously

Add the necessary hooks to the aic94xx driver to support the asynchronous SCSI
device scan infrastructure.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>