net: dwc_eth_qos: mdio: Implement clause 45

Bevor this commit, only clause 22 access was possible. After this commit,
clause 45 direct access will available as well.

Note that there is a slight change of behavior: Before this commit, the
C45E bit was set to whatever value was left in the register from the
previous access. After this commit, we adopt the common practice of
discerning C45 from C22 using the devad argument.

Signed-off-by: Philip Oberfichtner <pro@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>