Backport c289d6625237 ("Revert "pwm: Set class for exported channels in sysfs"") to 4.19

Hi,

Please consider backporting commit c289d6625237 ("Revert "pwm: Set class for exported channels in sysfs"") from 4.20 to 4.19 stable (the original buggy commit was introduced in 4.16).

This one-line revert fixes an oops triggered by writing to sysfs.

Thanks, John