Bluetooth: Remove useless l2cap_err_to_reason function

Now that the only reason code this function can return is
L2CAP_REJ_NOT_UNDERSTOOD we can just do the necessary assignment without
needing a separate function at all.

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