LPC546XX: Correct Ethernet MAC address write

Patch to LPC546XX SDK code - write the low Ethernet MAC address
register last, as that synchronises the update.

Without this change, the ENET_SetMacAddr call only seems to work prior
to MAC initialisation, causing problems for the new mbed OS EMAC system,
which expects it to be changable later.

Updated emac greentea tests #6851.

Author: kjbracey

targets/TARGET_NXP/TARGET_MCUXpresso_MCUS/TARGET_LPC546XX/drivers/fsl_enet.h

@@ -674,10 +674,10 @@ static inline void ENET_SetMacAddr(ENET_Type *base, uint8_t *macAddr)
 {
     assert(macAddr);
 
-    /* Set Macaddr */
+    /* Set Macaddr - low last as that synchronizes the update */
+    base->MAC_ADDR_HIGH = ((uint32_t)macAddr[5] << 8) | ((uint32_t)macAddr[4]);
     base->MAC_ADDR_LOW = ((uint32_t)macAddr[3] << 24) | ((uint32_t)macAddr[2] << 16) | ((uint32_t)macAddr[1] << 8) |
                          ((uint32_t)macAddr[0]);
-    base->MAC_ADDR_HIGH = ((uint32_t)macAddr[5] << 8) | ((uint32_t)macAddr[4]);
 }
 
 /*!