drivers: rtc: rtc_ds1307: Fix corruption of SECONDS register

We read/modify/write the CH/SECONDS register at initialisation to clear
only the Clock Halt bit and only if it is set.

The previous implementation zeroes the entire register unconditionally at
initialisation, which wipes the SECONDS fields.

Fixes #77354.

Signed-off-by: Andrew Feldhaus <github@feldhaus.co.uk>