pwm: core: Remove S_IFREG from debugfs_create_file()

The debugfs_create_file() already has a check and adds S_IFREG
automatically. Remove unneeded flag.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20221117165812.27757-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>