pwm: Register debugfs operations after the pwm class

While the debugfs operations don't technically depend on an initialized
class, they loop over the idr that only can get entries when the class
is properly initialized.

This also fixes the ugly (but harmless) corner case that the debugfs
file stays around after the pwm class failed to initialize.

While at it, add an appropriate error message when class initialization
fails.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20240626222529.2901200-2-u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>