pwm: sti: Use devm_kcalloc() instead of calculating the size for devm_kzalloc()

Using a multiplication to calculate the size of an allocation isn't
recommended in case the mulitplication overflows. While the chance this
happens is low, preventing such an error is easy enough; so do that.

Link: https://lore.kernel.org/r/17062aef42e6677629a056e25c6916d8b6eaedeb.1710068192.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>