bhnd(4): fix a few bugs in pwrctl/fixed-clock device support.

• Do not panic on siba(4) detach when the bhnd(4) bus calls bhnd_get_pmu_info() on a PMU-less device.
• Fix bhnd_pwrctl attach/detach on fixed-clock devices:
  • Treat bhnd_pwrctl_updateclk() as a no-op on fixed-clock devices.
  • Use bhnd_pwrctl_updateclk() to perform the appropriate clock transition on detach.

Sponsored by: The FreeBSD Foundation