watchdog: cpwd: fix build regression

The compat_ptr_ioctl() infrastructure did not make it into
linux-5.4, so cpwd now fails to build.

Fix it by using an open-coded version.

Fixes: 68f28b01 ("watchdog: cpwd: use generic compat_ptr_ioctl")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>