watchdog: f71808e_wdt: Add check for platform_driver_register

As platform_driver_register() could fail, it should be better
to deal with the return value in order to maintain the code
consisitency.

Fixes: 27e0fe00 ("watchdog: f71808e_wdt: refactor to platform device/driver pair")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
https://lore.kernel.org/r/20220526080303.1005063-1-jiasheng@iscas.ac.cn


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>