hp-wmi: split rfkill initialization out of hp_wmi_bios_setup

Split initialization of rfkill devices from hp_wmi_bios_setup() to
hp_wmi_rfkill_setup(). This makes the code somewhat cleaner, especially
with the future command 0x1b rfkill support.

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