rtc: stmp3xxx: Add failure handling for stmp3xxx_wdt_register()

Use platform_device_put() to free platform device before print
error message when platform_device_add() fails to run.

Fixes: 1a71fb84 ("rtc: stmp3xxx: add wdt-accessor function")
Signed-off-by: Lin Yujun <linyujun809@huawei.com>
Reviewed-by: Wolfram Sang <wsa@kernel.org>
Link: https://lore.kernel.org/r/20220915065253.43668-1-linyujun809@huawei.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>