clk: vt8500: Fix division-by-0 when requested rate=0

A request to vt8500_dclk_(round_rate/set_rate) with rate=0 results
in a division-by-0 in the kernel.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Mike Turquette <mturquette@linaro.org>