net: mvmdio: Use devm_* API to simplify the code

This commit makes use of devm_kmalloc_array() for memory allocation and the
recently introduced devm_mdiobus_alloc() API to simplify driver's code.
While here, remove a redundant out of memory error message.

Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>