dell-laptop: Pay attention to which devices the hardware switch controls

Right now, we assume that the hardware rfkill switch on Dells toggles all
radio devices. In fact, this can be configured in the BIOS and so right
now we may mark a device as hardware killed even when it isn't. Add code
to query the devices controlled by the switch, and use this when
determining the hardware kill state of a radio.

Signed-off-by: Matthew Garrett <mjg@redhat.com>