net: phy: fixed: return an error for Clause 45 over 22 reads

The fixed PHY driver does not properly emulate Clause 45 over Clause 22
MDIO reads, and as such, will return bogus values when we access such
registers.

Return an error when accessing these registers in order to prevent
advertising bogus capabilities such as EEE support and such.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>