src/include: Move assignment out of if condition

Fix the following error detected by checkpatch.pl:

ERROR: do not use assignment in if condition

TEST=Build and run on Galileo Gen2

Change-Id: I911d528bd85afcd9f3837241494f13d1f9f283ab
Signed-off-by: Lee Leahy <Leroy.P.Leahy@intel.com>
Reviewed-on: https://review.coreboot.org/18659
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/include/string.h

@@ -69,10 +69,13 @@ static inline char *strconcat(const char *s1, const char *s2)
 static inline char *strncpy(char *to, const char *from, int count)
 {
 	register char *ret = to;
+	register char data;
 
 	while (count > 0) {
 		count--;
-		if ((*to++ = *from++) == '\0')
+		data = *from++;
+		*to++  = data;
+		if (data == '\0')
 			break;
 	}