Fix adaptation interface unit tests (ARMmbed#2409)

Author: Jarkko Paso

test/nanostack/unittest/6LoWPAN/adaptation_interface/test_adaptation_interface.c

@@ -607,6 +607,7 @@ bool test_lowpan_adaptation_interface_indirect_tx()
 
     entry.mac_api = &api;
     api.mcps_data_req = NULL;
+    api.phyMTU = 127;
     socket_stub.buffer_ptr = NULL;
 
     entry.id = 0;
@@ -1316,6 +1317,7 @@ bool test_lowpan_adaptation_interface_mpx_register()
 
     entry.id = 0;
     api.mcps_data_req = &mcps_data_req_cb;
+    api.phyMTU = 127;
 
     test_buf2->ip_routed_up = true;
     protocol_core_stub.entry_ptr = &entry;