thunderbolt: Allow clearing the key

If secure authentication of a devices fails, either because the device
already has another key uploaded, or there is some other error sending
challenge to the device, and the user only wants to approve the device
just once (without a new key being uploaded to the device) the current
implementation does not allow this because the key cannot be cleared
once set even if we allow it to be changed.

Make this scenario possible and allow clearing the key by writing
empty string to the key sysfs file.

Signed-off-by: Yehezkel Bernat <yehezkel.bernat@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>