x86: touch_nmi_watchdog(): reset alert counters for supported nmi_watchdog modes only

The checking 'if nmi_watchdog > 0' (ie NMI_NONE) is quite fast but it
has a side effect - it's taken even if nmi_watchdog = NMI_DISABLED.

Nowadays nmi_watchdog is set up to NMI_NONE by default so this condition
is properly taken most the time but we better show this explicitly.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>