trivial: resolve compiler warning in ethdrivers

Change the check to require a non-empty string for the mdio bus name
instead of a redundant pointer check to an array condained within the
bus structure.

Signed-off-by: Kent McLeod <kent@kry10.com>

libethdrivers/src/plat/imx6/uboot/miiphyutil.c

diff --git a/libethdrivers/src/plat/imx6/uboot/miiphyutil.c b/libethdrivers/src/plat/imx6/uboot/miiphyutil.c
index ae8568d..c1fbcb7 100644
--- a/libethdrivers/src/plat/imx6/uboot/miiphyutil.c
+++ b/libethdrivers/src/plat/imx6/uboot/miiphyutil.c
@@ -162,7 +162,7 @@
 
 int mdio_register(struct mii_dev *bus)
 {
-    if (!bus || !bus->name || !bus->read || !bus->write) {
+    if (!bus || !bus->name[0] || !bus->read || !bus->write) {
         return -1;
     }