Extra newline.

SI_SUB_CONFIGURE is fine for this purpose.

I think we probably want to ensure that the device file was opened for writing, at least for this ioctl: check (fflags & FWRITE) != 0.

enums shouldn't be used in binary interfaces, I believe C doesn't provide guarantees about the width of an enum type. It should be unsigned int or uint32_t.

I would hoist this out of the specific case, it's a bit safer to check for FWRITE by default and bypass it for some select few ioctls.

Probably you want to check that trans is a valid enum member, since power_pm_suspend() does not return an error.

yeah, in fact, looking at where power_pm_suspend() is called, it probably makes sense to make the default: panic

Nitpicky, but IMO it's better to check the bounds before assigning to an enum variable, rather than implicitly assuming that the two types are the same width. Something like:

uint32_t trans;
...
trans = *(uint32_t)data;
if (trans >= POWER_SSTATE_TRANSITION_COUNT)
    return (EINVAL);
power_pm_suspend((enum power_sstate_transition)trans);