enetc: Clean up local mdio bus allocation

What's needed is basically a pointer to the mdio registers.
This is one way to store it inside bus->priv allocated space,
without upsetting sparse.
Reworked accessors to avoid __iomem casting.
Used devm_* variant to further clean up the init error /
remove paths.

Fixes following sparse warning:
 warning: incorrect type in assignment (different address spaces)
    expected void *priv
    got struct enetc_mdio_regs [noderef] <asn:2>*[assigned] regs

Fixes: ebfcb23d ("enetc: Add ENETC PF level external MDIO support")

Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>