intel_hwpstate(4): Don't leak bound thread in error conditions

I don't know why a Skylake CPU with the HWP feature bit present would trap
on MSR reads of the HWP registers, but if this occurs, do not leave the
attach thread bound. This could conceivably cause reported hangs, although
I have no evidence that this is the cause.

Reported by: ae@, Andreas Nilsson <andrnils AT gmail.com>
X-MFC-With: r357002