Bluetooth: debugfs option to unset MITM flag

The BT qualification test SM/MAS/PKE/BV-01-C needs us to turn off
the MITM flag when pairing, and at the same time also set the io
capability to something other than no input no output.

Currently the MITM flag is only unset when the io capability is set
to no input no output, therefore the test cannot be executed.

This patch introduces a debugfs option to force MITM flag to be
turned off.

Signed-off-by: Archie Pusaka <apusaka@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>