apps/blestress: set BLE_ACL_BUF_SIZE to proper value

With current (default) CoC MPS setting we were trying to send more
than HCI packet could hold. This patch increases its size to sufficient
value.
diff --git a/apps/blestress/syscfg.yml b/apps/blestress/syscfg.yml
index 4f7fa4d..b242804 100644
--- a/apps/blestress/syscfg.yml
+++ b/apps/blestress/syscfg.yml
@@ -82,3 +82,6 @@
 
     # Whether to save data to sys/config, or just keep it in RAM.
     BLE_STORE_CONFIG_PERSIST: 0
+
+    # ACL buf size must be able to contain CoC MPS plus ACL header
+    BLE_TRANSPORT_ACL_SIZE: MYNEWT_VAL_BLE_L2CAP_COC_MPS + 4