dell-rbtn: Export notifier for other kernel modules

This patch exports notifier functions so other modules can receive HW
switch events. By default when some module register notifier, dell-rbtn
driver automatically remove rfkill interfaces from system (it is expected
that other module will use events for other rfkill interface). This
behaviour can be changed with new module parameter "auto_remove_rfkill".

This patch is designed for dell-laptop module for receiving those events.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
[dvhart@linux.intel.com: Cleanup MODULE_PARM_DESC formatting and grammar]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>