watchdog: atcwdt200: fix return value when watchdog is enabled

watchdog: atcwdt200: fix return value when watchdog is enabled

atcwdt_get_int_timer_type() returned TMR_UNKNOWN (enum value 3) when
CTRL_WDT_EN was set in the control register.

Because atcwdt_probe() checks "ret = atcwdt_get_int_timer_type(drv_data);
if (ret) return ret;", returning TMR_UNKNOWN caused probe() to return 3
instead of a valid negative error code.

Fix this by returning -EBUSY via dev_err_probe() when the watchdog is
already enabled and update the kernel-doc description accordingly.

Fixes: e4e0848ad046 ("watchdog: atcwdt200: Add driver for Andes ATCWDT200")
Signed-off-by: Surendra Singh Chouhan <kr494167@gmail.com>
Link: https://lore.kernel.org/r/20260723022634.8642-1-kr494167@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>