acpi: Use M_WAITOK in acpi_register_ioctl

[jhb]

Summary

This function is only called from device attach routines which can
sleep.

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[jhb]

BTW, I've thought about adding an acpi_unregister_ioctl_all or the like that just takes the fn argument and unregisters all commands that use the same function. That would be a nicer way to handle unloading of a module that registers multiple handlers.