hp-wmi: clear rfkill device pointers when appropriate

NULLify rfkill pointers during initialization. This prevents dereference
of invalid pointer in case the driver is rebound and some rfkill device
isn't detected anymore. Clear them also in hp_wmi_rfkill_setup failure
path so that an rfkill initialization failure doesn't need to be fatal
for the whole driver.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Matthew Garrett <mjg@redhat.com>