pwm: sysfs: Replace sprintf() with sysfs_emit()

While the uses in this code are unproblematic, it's generally safer for
sysfs outputs to use the new sysfs_emit() helper instead of raw calls to
sprintf() and friends. This also has the benefit of annotating the uses,
which makes them easier to audit and potentially use them to generate
sysfs documentation from them.

This patch replaces existing sprintf() calls straightforwardly with the
new helper.

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