pwm: core: Use octal permission

Permission bits are easier readable in octal than with using the
symbolic names.

Fixes the following warning generated by checkpatch:

    WARNING: Symbolic permissions 'S_IRUGO' are not preferred. Consider using octal permissions '0444'.
    #1341: FILE: drivers/pwm/core.c:1341:
    +       debugfs_create_file("pwm", S_IFREG | S_IRUGO, NULL, NULL,

Signed-off-by: Soham Biswas <sohambiswas41@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>