net: stmmac: dwmac-visconti: No change to ETHER_CLOCK_SEL for unexpected speed request.

Variable clk_sel_val is not initialized in the default case of the first switch statement.
In that case, the function should return immediately without any changes to the hardware.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
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>