nimble/host: Truncate data in ble_eatt_tx

Data packets sent over eatt chan weren't correctly truncated.
Data packet is now adjusted to peer device's MTU for the channel.
Fixes various GATT/SR test cases that verify support for Read Long
Characteristic Value Requests, Multiple Read Characteristic Value
Requests etc.

nimble/host/src/ble_eatt.c

@@ -476,6 +476,8 @@ ble_eatt_tx(uint16_t conn_handle, uint16_t cid, struct os_mbuf *txom)
  goto error;
  }
 
+ ble_att_truncate_to_mtu(eatt->chan, txom);
+
  rc = ble_l2cap_send(eatt->chan, txom);
  if (rc == 0) {
  goto done;