pwm: fsl-ftm: Handle clk_get_rate() returning 0

pwm: fsl-ftm: Handle clk_get_rate() returning 0

Considering that the driver doesn't enable the used clocks (and also
that clk_get_rate() returns 0 if CONFIG_HAVE_CLK is unset) better check
the return value of clk_get_rate() for being non-zero before dividing by
it.

Fixes: 3479bbd1e1f8 ("pwm: fsl-ftm: More relaxed permissions for updating period")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/b68351a51017035651bc62ad3146afcb706874f0.1743501688.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>