watchdog: rti-wdt: Fix pm_runtime_get_sync() error checking

If the device is already in a runtime PM enabled state
pm_runtime_get_sync() will return 1, so a test for negative
value should be used to check for errors.

Fixes: 2d63908b

 ("watchdog: Add K3 RTI watchdog support")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220412070824.23708-1-linmq006@gmail.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>