watchdog: core: fix WDIOC_GETSTATUS return value

watchdog: core: fix WDIOC_GETSTATUS return value

In commit 237744c17b752d07bf232afaf40daaa507d4b2b6 we added
a wrapper for the WDIOC_GETSTATUS ioctl call. The code results
however in a different behaviour: it returns an error if the
driver doesn't support the status operation. This is not
according to the API that says that when we don't support
the status operation, that we just should return a 0 value.
Only when the device isn't there anymore, we should return an
error.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>