I can see the point of having a sysfs attribute to enable MMCONF from userspace, so
that userland diagnostics tools can turn it on if they really really want to.
(I'd make that printk a nice warning "application XYZ is enabling extended config space for devize ABC" so
that if the box then crashes and burns, people know who/why and where to direct their emails ;-)
We did something similar for "enable", it's maybe 10 lines of code or so.
I would assume lspci and friends would then only turn that on at explicit admin request