dell-laptop: Use dell-rbtn instead i8042 filter when possible

Until now module dell-laptop registered rfkill device which used i8042
filter function for receiving HW switch rfkill events (handling special
keycode).

But for some dell laptops there is native ACPI driver dell-rbtn which can
receive rfkill events (without i8042 hooks).

So this patch will combine best from both sides. It will use native ACPI
driver dell-rbtn for receiving events and dell-laptop SMBIOS interface for
enabling or disabling radio devices. If ACPI driver or device will not be
available fallback to i8042 filter function will be used.

Patch also changes module_init() to late_initcall() to ensure that init
function will be called after initializing dell-rbtn.c driver.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>