Problem with snd_mss driver probe/attach, maybe ACPI conflict?

I have an MSS-compatible ISA sound card using the snd_mss driver.  When I
boot with snd_mss_load="YES" in /boot/loader.conf, the acpi_cpu0 device
doesn't attach and I get 14 errors like this:

pcm0: <CS4231> port 0x530-0x537 on acpi0
device_probe_and_attach: pcm0 attach returned 6

And pcm0 attaches normally:

pcm0: <Yamaha OPL-SAx> at port
0x370-0x371,0x330-0x331,0x388-0x38b,0x530-0x537,0x220-0x22f irq 5 drq 1,0 on
isa0

If I boot the system without the driver, acpi_cpu0 attaches:

acpi_cpu0: <CPU> on acpi0
acpi_cpu: throttling enabled, 8 steps (100% to 12.5%), currently 100.0%

Then, kldloading snd_mss.ko after multi-user startup, I get no errors and
pcm0 attaches normally:

pcm0: <Yamaha OPL-SAx> at port
0x370-0x371,0x330-0x331,0x388-0x38b,0x530-0x537,0x220-0x22f irq 5 drq 1,0 on
isa0

In both cases the sound card works for PCM playback, which is all the
functionality I need.  This reminds me of probe-order problems from way
back.  How do I tell FreeBSD to attach acpi_cpu0? before trying to attach
pcm0?  If that's not what I need to do, how do I fix this?

This is on 5.2.1-p9.