drivers: watchdog: atcwdt200: fix shadow variable build error

Fix a shadow variable build error:

wdt_andes_atcwdt200.c:112:49: warning: declaration of 'counter_dev'
shadows a global declaration [-Wshadow]
  112 | static void wdt_counter_cb(const struct device *counter_dev,
      uint8_t chan_id, |
~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~ wdt_andes_atcwdt200.c:95:35: note:
shadowed declaration is here
   95 | static const struct device *const counter_dev = |
      ^~~~~~~~~~~

Make the outer variable more specific rather than the local one.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>