net: execute "miiphy_init" if CONFIG_PHYLIB defined

In "common/Makefile" "miiphyutil.o" gets built if any of the following
items enabled:
 * CONFIG_PHYLIB
 * CONFIG_MII
 * CONFIG_CMD_MII

So it's possible to not define CONFIG_MII or CONFIG_CMD_MII and still
use functions like "miiphy_get_dev_by_name".

In its turn "miiphy_get_dev_by_name" traverses "mii_devs" list which is
not initialized because "miiphy_init" never got called.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Wolfgang Denk <wd@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>