pwm: tiehrpwm: Make code comment in .free() more useful

pwm: tiehrpwm: Make code comment in .free() more useful

Instead of explaining trivia to everyone who can read C describe the
higher-level effect of setting pc->period_cycles[pwm->hwpwm] to zero.

Fixes: 01b2d4536f02 ("pwm: pwm-tiehrpwm: Fix conflicting channel period setting")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/4c38dd119a77d7017115318a3f2c50bde62a6f21.1754927682.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>