Exported pwm channels aren't removed before the pwmchip and are
leaked. This results in invalid sysfs files. This fix removes
all exported pwm channels before chip removal.
Signed-off-by: David Hsu <davidhsu@google.com>
Fixes: 76abbdde ("pwm: Add sysfs interface")
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>