ar71xx: ag71xx: use mdio bus name in ar7240_probe messages

The ar7240_probe function uses the network device name
in the kernel log messages, however the name is not yet
initialized when the ar7240_probe function is called.
Use the mdio bus name in the messages to avoid ugly
log lines like the following one:

  eth%d: Found an AR7240/AR9330 built-in switch

Reported-by: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39116