rtc: pcf2127: use sysfs_emit() instead of sprintf()

Use sysfs_emit() instead of sprintf() in the sysfs show callback
(timestamp_show) to conform to the recommended sysfs attribute
formatting guidelines in Documentation/filesystems/sysfs.rst.

Reviewed-by: Bruno Thomsen <bruno.thomsen@gmail.com>
Link: https://patch.msgid.link/20261029132339.925021-2-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>