The following line should work for the lower level driver in /etc/modules.conf:
alias scsi_hostadapter aic7xxx
Personally, I build the mid level support (scsi_mod) into the kernel. This module would have to be loaded before the upper (tape drive/disk) or lower (adapter) support. I suspect that when you load the aic7xxx module, it might force the loading of scsi_mod if it is not already present, but perhaps the following might force it:
add below aix7xxx scsi_mod
This specifies that if the aix7xxx module is needed, the scsi_mod will be loaded as well, and first.
Then again, I understand it is the scsi_mod module that requests the scsi_hostadapter so results are of course not guaranteed.