Bluetooth: Add clarifying comment for conn->auth_type

When responding to an IO capability request when we're the initiators of
the pairing we will not yet have the remote IO capability information.
Since the conn->auth_type variable is treated as an "absolute"
requirement instead of a hint of what's needed later in the user
confirmation request handler it's important that it doesn't have the
MITM bit set if there's any chance that the remote device doesn't have
the necessary IO capabilities.

This patch adds a clarifying comment so that conn->auth_type is left
untouched in this scenario.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>