rfkill: Use switch to demux userspace operations

Using a switch to handle different ev.op values in rfkill_fop_write()
makes the code easier to extend, as out-of-range values can always be
handled by the default case.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
[roll in fix for RFKILL_OP_CHANGE from Jouni]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>