scsi: aic7xxx: fix firmware build with O=path

Building firmware with O=path was apparently broken in aic7 for ever.
Message of the previous commit to the Makefile (from 2008) mentions this
unfortunate state of affairs already.  Fix this, mostly to make
randconfig builds more reliable.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>