tile: Enable NMIs on return from handle_nmi() without errors

NMI interrupts mask ALL interrupts before calling the handler,
so we need to unmask NMIs according to the value handle_nmi() returns.
If it returns zero, the NMIs should be re-enabled; if it returns
a non-zero error, the NMIs should be disabled.

Signed-off-by: Zhigang Lu <zlu@tilera.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>