[MIPS] au1x00_eth.c: Fixed a warning on pb1000 build.

au1x00_eth.c: In function 'au1x00_miiphy_write':
au1x00_eth.c:139: warning: 'return' with no value, in function returning non-void

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

cpu/mips/au1x00_eth.c

@@ -136,7 +136,7 @@ int  au1x00_miiphy_write(char *devname, unsigned char addr,
 		udelay(1000);
 		if (--timedout == 0) {
 			printf("au1x00_eth: miiphy_write busy timeout!!\n");
-			return;
+			return -1;
 		}
 	}