[SCSI] aic7xxx: fix firmware build

This patch adds the proper $(obj) and $(src) prefixes to dependency
rules in aic7xxx makefile. Without this patch, there is a remote
possibility that parallel make with a different output directory can
fail.

Also changed the deprecated EXTRA_CFLAGS construct to ccflags-y syntax.

Fixed up patch to survive "make drivers/scsi/ -j"
with BUILD_FIRMWARE enable. /Sam

Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>