rfkill: remove too-strict __must_check

Some drivers don't need the return value of rfkill_set_hw_state(),
so it should not be marked as __must_check.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>