net: phy: allow Clause 45 access via mii ioctl

Allow userspace to generate Clause 45 MII access cycles via phylib.
This is useful for tools such as mii-diag to be able to inspect Clause
45 PHYs.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>