dell-laptop: Add debugfs support

Export the status of RF killswitch through debugfs.

The killswitch status is obtained by the SMI to BIOS. Exporting this status
through debugfs can help identify the issue with the misbehaving firmware.

Signed-off-by: Keng-Yu Lin <keng-yu.lin@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>