asmc: do not drop shutdown notification when the system node fails

[seuros]

Summary

asmc_attach() registered the MBSS shutdown-cause handler
(EVENTHANDLER_REGISTER(shutdown_pre_sync, asmc_shutdown, ...))
after attempting to add the dev.asmc.N.system sysctl subtree. If that
subtree registration failed, the error path took goto nosms, jumping clean
over the shutdown-handler registration. On T2 Macs this silently drops
the only thing standing between a clean shutdown and bridgeOS's
watchdog power-cycling the machine.

MFC after: 3 days

[ngie]

A bit more context would help the layman: why is this change needed?

[seuros]

https://gist.github.com/seuros/c83b357a2c8c652e3e09016015dd6245

[ngie]

In D61482#1409597, @seuros wrote:

https://gist.github.com/seuros/c83b357a2c8c652e3e09016015dd6245

That's a great explanation -- please use that :)!

[ngie]

sys/dev/asmc/asmc.c

277–279

Should this go above asmc_sysctl_add_fan on line 249, just to be safe?

281–283

If the return value is going to be ignored, does it make more sense to convert the function to a void return for explicitness? What do we lose from ignoring this information?

284–327

This feels like it should all be under a secondary helper function like _asmc_attach_sms(..) for explicitness and to help bound the logic that is needed for platforms that have SMS capability.
This doesn't need to be acted on now. Just something to open for discussion and maybe do at a later date.