[MIPS] gth2.c: Fix a warning on gth2 build.

gth2.c: In function 'misc_init_r':
gth2.c:434: warning: pointer targets in passing argument 2 of 'setenv' differ in signedness

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>

board/gth2/gth2.c

@@ -431,7 +431,7 @@ int misc_init_r(void){
 	    (Rx[8] != ':') | (Rx[11] != ':') | (Rx[14] != ':')) {
 		printf ("*** ethernet addr invalid, using default ***\n");
 	} else {
-		setenv ("ethaddr", Rx);
+		setenv ("ethaddr", (char *)Rx);
 	}
 	return (0);
 }