iio: trigger: stm32-timer: Convert sysfs sprintf/snprintf family to sysfs_emit

Fix the following coccicheck warning:
drivers/iio/trigger/stm32-timer-trigger.c:299:8-16: WARNING:
use scnprintf or sprintf

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1618216751-1678-4-git-send-email-tiantao6@hisilicon.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>