Bluetooth: GATT: Be consistent with error in case SMP is disabled

This makes the code consistent with respect of errors, so instead of
checking directly on bt_gatt_write_without_response let this up to
bt_smp_sign.

Change-Id: Iea8d0bd2020df427b7542e2878ce8d9fd8b94170
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>