pwm: atmel-hlcdc: Prevent division by zero

The slow and system clock should never return a rate of zero, but this
might happen if the clocks property defined in the DT is referencing the
wrong clocks.

Prevent any division by zero from happening by testing the clk_freq
value before calling do_div().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>