pwmtestperf: Implement -I for inversed polarity requests

The driver handling for .duty_offset_ns > 0 isn't always trivial to get
right. So make pwmtestperf capable to do some tests with a non-zero duty
offset.

Fix a typo in the manpage en passant.

Link: https://lore.kernel.org/r/20260415143724.447340-2-u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>