dp83640: Ensure against premature access to PHY registers after reset

The datasheet specifies a 3uS pause after performing a software
reset. The default implementation of genphy_soft_reset() does not
provide this, so implement soft_reset with the needed pause.

Signed-off-by: Esben Haabendal <eha@deif.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>