netdev-linux: Avoid "cleverness" in swap_uint64().

Obviously correct code is easier on everyone.  As the C FAQ says:

20.15c:	How can I swap two values without using a temporary?

A:	The standard hoary old assembly language programmer's trick is:

		a ^= b;
		b ^= a;
		a ^= b;

	But this sort of code has little place in modern, HLL
	programming.  Temporary variables are essentially free,
	and the idiomatic code using three assignments, namely

		int t = a;
		a = b;
		b = t;

	is not only clearer to the human reader, it is more likely to be
	recognized by the compiler and turned into the most-efficient
	code (e.g. using a swap instruction, if available).  The latter
	code is obviously also amenable to use with pointers and
	floating-point values, unlike the XOR trick.  See also questions
	3.3b and 10.3.

lib/netdev-linux.c

@@ -1144,9 +1144,9 @@ netdev_linux_update_is_pseudo(struct netdev_dev_linux *netdev_dev)
 static void
 swap_uint64(uint64_t *a, uint64_t *b)
 {
-    *a ^= *b;
-    *b ^= *a;
-    *a ^= *b;
+    uint64_t tmp = *a;
+    *a = *b;
+    *b = tmp;
 }
 
 /* Retrieves current device stats for 'netdev'. */