net: ethernet: renesas: ravb_main: test clock rate to avoid division by 0

The clk API may return 0 on clk_get_rate, so we should check the result before
using it as a divisor.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>