phy: improve safety of fixed-phy MII register reading

There is no prevention of a concurrent call to both fixed_mdio_read()
and fixed_phy_update_state(), which can result in the state being
modified while it's being inspected.  Fix this by using a seqcount
to detect modifications, and memcpy()ing the state.

We remain slightly naughty here, calling link_update() and updating
the link status within the read-side loop - which would need rework
of the design to change.

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