rtc: ds1685: use sysfs_emit() instead of sprintf()

Use sysfs_emit() instead of sprintf() in sysfs show callbacks
(ds1685_rtc_sysfs_battery_show, ds1685_rtc_sysfs_auxbatt_show and
ds1685_rtc_sysfs_serial_show) to conform to the recommended sysfs
attribute formatting guidelines in
Documentation/filesystems/sysfs.rst.

Acked-by: Joshua Kinard <linux@kumba.dev>
Link: https://patch.msgid.link/20261029132339.925021-9-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>