net: stmmac: dwmac-visconti: Fix clock configuration for RMII mode

Bit pattern of the ETHER_CLOCK_SEL register for RMII/MII mode should be fixed.
Also, some control bits should be modified with a specific sequence.

Fixes: b38dd98f ("net: stmmac: Add Toshiba Visconti SoCs glue driver")
Signed-off-by: Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>
Reviewed-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>