net: mvmdio: check the MII_ADDR_C45 bit is not set for smi operations

Add a check for the read and write smi operations, to ensure the
MII_ADDR_C45 bit isn't set. This will be needed as soon as the xSMI
support is added to the mvmdio driver.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>