watchdog: xen: don't clear is_active when xen_wdt_stop() failed

xen_wdt_release() shouldn't clear is_active even when the watchdog
didn't get stopped (which by itself shouldn't happen, but let's return
a proper error in this case rather than adding a BUG() upon hypercall
failure).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>