Init MAC MTU size based on driver MTU size (ARMmbed#2397)

Initialise MAC MTU size based on driver MTU size. In simulator MTU
size was not properly updated to adaptation_interface by using
`mac802_15_4Mode` change as adaptation_interface was already created
in `arm_nwk_interface_lowpan_init`.

source/MAC/IEEE802_15_4/sw_mac.c

@@ -102,9 +102,9 @@ mac_api_t *ns_sw_mac_create(int8_t rf_driver_id, mac_description_storage_size_t
 
     // Set default MTU size to 127 unless it is too much for PHY driver
     if (driver->phy_driver->phy_MTU > MAC_IEEE_802_15_4_MAX_PHY_PACKET_SIZE) {
-        this->phyMTU = MAC_IEEE_802_15_4_MAX_PHY_PACKET_SIZE;
-    } else {
         this->phyMTU = driver->phy_driver->phy_MTU;
+    } else {
+        this->phyMTU = MAC_IEEE_802_15_4_MAX_PHY_PACKET_SIZE;
     }
 
     mac_store.setup = mac_mlme_data_base_allocate(mac_store.dev_driver->phy_driver->PHY_MAC, mac_store.dev_driver, storage_sizes, this->phyMTU);