Bluetooth: ATT: Set correct MTU for EATT L2CAP channel

The EATT MTU in Zephyr is static. We know it at channel creation time,
so we should communicate the MTU as part of channel creation.

Side note: With this approach, it should no longer be neccessary or
useful to do a channel reconfigure.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>

subsys/bluetooth/host/Kconfig.l2cap

@@ -31,6 +31,7 @@ config BT_L2CAP_TX_MTU
 	int "Maximum supported L2CAP MTU for L2CAP TX buffers"
 	default 253 if BT_BREDR
 	default 69 if BT_MESH_GATT
+	default 66 if BT_EATT
 	default 65 if BT_SMP
 	default 64 if BT_BAP_UNICAST_SERVER || \
 		      BT_BAP_UNICAST_CLIENT || \
@@ -39,6 +40,7 @@ config BT_L2CAP_TX_MTU
 		      BT_BAP_BROADCAST_ASSISTANT
 	default 49 if BT_HAS || BT_HAS_CLIENT
 	default 23
+	range 66 2000 if BT_EATT
 	range 65 2000 if BT_SMP
 	range 23 2000
 	help

subsys/bluetooth/host/att.c

@@ -3250,7 +3250,16 @@ static struct bt_att_chan *att_chan_new(struct bt_att *att, atomic_val_t flags)
 	chan->att = att;
 	att_chan_attach(att, chan);
 
-	if (!bt_att_is_enhanced(chan)) {
+	if (bt_att_is_enhanced(chan)) {
+		/* EATT: The MTU will be sent in the ECRED conn req/rsp PDU. The
+		 * TX MTU is received on L2CAP-level.
+		 */
+		chan->chan.rx.mtu = BT_LOCAL_ATT_MTU_EATT;
+	} else {
+		/* UATT: L2CAP Basic is not able to communicate the L2CAP MTU
+		 * without help. ATT has to manage the MTU. The initial MTU is
+		 * defined by spec.
+		 */
 		chan->chan.tx.mtu = BT_ATT_DEFAULT_LE_MTU;
 		chan->chan.rx.mtu = BT_ATT_DEFAULT_LE_MTU;
 	}