common: miiphyutil: avoid memory leak

The following code will alloc memory for new_dev and ldev:
"
new_dev = mdio_alloc();
ldev = malloc(sizeof(*ldev));
"
Either new_dev or ldev is NULL, directly return, but this may leak memory.
So before return, using free(ldev) and mdio_free(new_dev) to avoid
leaking memory, also free can handle NULL pointer.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
(cherry picked from commit 746da1bd42aa5ecc47898399514c9c76d0329706)