pwm: stm32: Fix error message to not describe the previous error path

"Failed to lock the clock" is an appropriate error message for
clk_rate_exclusive_get() failing, but not for the clock running too
fast for the driver's calculations.

Adapt the error message accordingly.

Fixes: d44d6356

 ("pwm: stm32: Fix for settings using period > UINT32_MAX")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/285182163211203fc823a65b180761f46e828dcb.1718979150.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>