host/ble_att.c: fix ATT MTU for EATT channels

For EATT bearer, ATT MTU is same for bot RX and TX and is minimum of the
two. L2CAP Credit Based Reconfigure Request can be used to change its
value.

nimble/host/src/ble_att.c

@@ -463,9 +463,12 @@ ble_att_chan_mtu(const struct ble_l2cap_chan *chan)
     uint16_t mtu;
 
 #if MYNEWT_VAL(BLE_EATT_CHAN_NUM) > 0
-    if (chan->scid != BLE_L2CAP_CID_ATT) {
-        /* EATT case */
-        return chan->coc_tx.mtu;
+    if (chan->psm == BLE_EATT_PSM) {
+        /* The ATT_MTU for the Enhanced ATT bearer shall be set to the minimum of the
+         * MTU field values of the two devices. Reference:
+         * Core v5.0, Vol 6, Part B, section 1.3.2.1
+         */
+        return min(chan->coc_tx.mtu, chan->coc_rx.mtu);
     }
 #endif