watchdog: core: propagate ping error code to the user space

Watchdog ping return errors are ignored by watchdog core,
Whatchdog daemon should be informed about possible hardware error or
underlaying device driver get unregistered.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>