dell-laptop: Don't read-back sw_state on machines with a hardware switch

On machines with a hardware switch, the blocking settings can not be changed
through a Fn + wireless-key combo, so there is no reason to read back the
blocking state from the BIOS.

Reading back is not only not necessary it is actually harmful, since on some
machines the blocking state will be cleared to all 0 after a wireless switch
toggle, even for radios not controlled by the hw-switch (yeah firmware bugs).

This causes "magic" changes to the sw_state. This is inconsistent with other
rfkill drivers which preserve the sw_state over a hw kill on / off.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>