MPS2 CM3DS ethernet fix heap bug

The function SMSC9220_EMAC::low_level_input should create a heap for the
packet equal to the size of the message (most of which are couple hundred
bytes). The current code uses maximum frame size (1522 bytes) for each
packet. This will cause the heap to quickly fill up. In fact, the default
memory size (lwip.mem-size) used for this heap is 1600 bytes. This means
that once you have one other packet allocated (extremely common), the
heap allocation will always fails.

Also, it is recommend to increase the default lwip.mem-size because that
amount is very small especially if you send or receive a few large packets
in the network. This is NOT done in current commit.

connectivity/drivers/emac/TARGET_ARM_SSG/COMPONENT_SMSC9220/smsc9220_emac.cpp

@@ -89,7 +89,7 @@ emac_mem_buf_t *SMSC9220_EMAC::low_level_input()
         message_length -= CRC_LENGTH_BYTES;
     }
 
-    p = _memory_manager->alloc_heap(SMSC9220_ETH_MAX_FRAME_SIZE,
+    p = _memory_manager->alloc_heap(message_length,
                                     SMSC9220_BUFF_ALIGNMENT);
 
     if (p != NULL) {