[devices]: Seastone fix issue #18767 psu fan speed issue (#18787)

Fix issue #18787 fan information could not been shown issue.

How I did it
Add psu fan get_target_speed api support.

How to verify it
Use `show platform fan` in branch 202311.

device/celestica/x86_64-cel_seastone-r0/sonic_platform/fan.py

@@ -188,6 +188,8 @@ def get_target_speed(self):
             sysfs_path = sysfs_path.format(fan_index[self.fan_tray_index])
             pwm = self._api_helper.read_txt_file(sysfs_path)
             target = round(int(pwm) / 255 * 100.0)
+        else:
+            return self.get_speed()
 
         return target