net: dsa: fix warning same module names

When building with CONFIG_NET_DSA_REALTEK_SMI and CONFIG_REALTEK_PHY
enabled as loadable modules, we see the following warning:

warning: same module names found:
  drivers/net/phy/realtek.ko
  drivers/net/dsa/realtek.ko

Rework so the driver name is realtek-smi instead of realtek.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>