net/usb: add relative mii functions for r815x

Base on cdc_ether, add the mii functions for RTL8152 and RTL8153.
The RTL8152 and RTL8153 support ECM mode which use the driver of
cdc_ether. Add the mii functions. Then, the basic PHY access is
possible.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>