net: designware: fix PHY reset with DM_MDIO

The dw_eth_pdata is not accessible from the mdio device, it gets the mdio bus plat
leading to random sleeps (-10174464 on Odroid-HC4).

This moves the dw_mdio_reset function to a common one taking the ethernet
device as parameter and use it from the dw_mdio_reset and dm_mdio variant functions.

Fixes: 5160b456 ("net: designware: add DM_MDIO support")
Reported-by: Mark Kettenis <mark.kettenis@xs4all.nl>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>