watchdog: s3c2410_wdt: Assign s3c_wdt[] until probe complete

Assigns device data to `s3c_wdt[cluster_index]` only when probe function
completes. Several functions of s3c2410_wdt use the existence of
`s3c_wdt[*]` to decide whether the device data is ready to be accessed.
This causes an invalid access issue as long as the probe function puts
device data to `s3c_wdt[cluster_index]` before completely preparing the
content. Fixes the issue by rearranging the assignment order.

Bug: 342585125
Change-Id: Idb4c3b71fb2e0518725c697db01e708aa0c7c86b
Signed-off-by: Woody Lin <woodylin@google.com>
(cherry picked from commit d7bd15571d51e658a081d98dfbcc17e3aa104585)