nimble/ll: Require 257 bytes HCI events when enabling fake dual-mode

Some BR/EDR commands eg. Read Local Name command have fixed
(max allowed HCI event size) return values. If event size is
smaller it results in assert.

nimble/controller/syscfg.hbd.yml

@@ -23,3 +23,5 @@ syscfg.defs:
             device. It can be initialized and used by Windows 10 to some
             extent.
         value: 0
+        restrictions:
+           - '(BLE_TRANSPORT_EVT_SIZE == 257) if 1'