pwm: lpss: Add a comment to the bypass field

Add a comment to the bypass field based on the commit b997e3ed
("pwm: lpss: Set enable-bit before waiting for update-bit
to go low").

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>