drivers: rtc: ds3231: fix uninitialized variable error

Until recently this driver wasn't built in CI. Now that it is, clang
is complaining about `err` possibly being uninitialized in case
`bitmask` is 255. Fix it by actually removing code that was
unnecessarily trying to access the uninitialized variable.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>