rfkill: fix miscdev ops

The /dev/rfkill ops don't refer to the module,
so it is possible to unload the module while
file descriptors are open. Fix this oversight.

Reported-by: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>