sys/modules: fix bogus OPT_ACPI tests

ACPI is not handled specially by sys/conf/kern.opts.mk (unlike a few
options), so we should fall back on the generic behavior of
sys/conf/config.mk, which pulls from all the generated opt*.h files,
including opt_acpi.h, which will cause DEV_ACPI to be included in
KERN_OPTS. Then the generic machinery in sys/conf/kmod.mk will cause
SRCS.DEV_ACPI to be included in SRCS when appropriate.

Reviewed by: jhb, imp
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D38737