scsi: fix makefile for aic7(3*x)

Fix bug introduced by my latest fix to the aic7xxx Makefile.
Test build on x86 32 and 64 bit.
Without and with -j (parallel build)
Building firmaware is br0ken with O=... but this
is unrelated to this bug-fix.

Tested-by: Adrian Bunk <bunk@kernel.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>