siba(4): Add support for devices have neither PMU nor PWRCTL support.

Very early BHND Wi-Fi devices (e.g. BCM4318) do not support any form of
dynamic clock control; on these devices, any PMU requests that cannot be
met by the device's fixed clock state will return an appropriate error
code.

Approved by: adrian (mentor, implicit)
Sponsored by: The FreeBSD Foundation