initialize all members of gattc_write_params_t in GattClient::write().

This avoids some compiler warnings about un-initialized members.

source/nRF5xGattClient.h

@@ -122,9 +122,11 @@ public:
     }
 
     virtual ble_error_t write(GattClient::WriteOp_t cmd, Gap::Handle_t connHandle, GattAttribute::Handle_t attributeHandle, size_t length, const uint8_t *value) const {
-        ble_gattc_write_params_t writeParams = { };
+        ble_gattc_write_params_t writeParams;
         writeParams.write_op = cmd;
+        writeParams.flags    = 0; /* this is inconsequential */
         writeParams.handle   = attributeHandle;
+        writeParams.offset   = 0;
         writeParams.len      = length;
         writeParams.p_value  = const_cast<uint8_t *>(value);