Change GATT_ERROR to GATT_REQ_NOT_SUPPORTED

GATT_ERROR occurs in GATTC_ConfigureMTU when the link is not BLE, which
is covered by existing status code GATT_REQ_NOT_SUPPORTED. Thus changing
to correct status code.

Bug: 201571634
Test: m com.android.btservices
Flag: EXEMPT, no flow change
Change-Id: I141b826235c887b19334d7c389f533d7226a8d42