pwm: mediatek: Fix various issues in the .apply() callback

pwm: mediatek: Fix various issues in the .apply() callback

duty_cycle and period were silently cast from u64 to int losing
relevant bits. Dividing by the result of a division (resolution) looses
precision. clkdiv was determined using a loop while it can be done
without one. Also too low period values were not catched.

Improve all these issues. Handling period and duty_cycle being u64 now
requires a bit more care to prevent overflows, so mul_u64_u64_div_u64()
is used.

The changes implemented in this change also align the chosen hardware
settings to match the usual PWM rules (i.e. round down instead round
nearest) and so .apply() also matches .get_state() silencing several
warnings with PWM_DEBUG=y. While this probably doesn't result in
problems, this aspect makes this change---though it might be considered
a fix---unsuitable for backporting.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250725154506.2610172-16-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>