xtensa: define NO_IRQ appropriately for the architecture

Interrupt number zero (0) is valid for the Xtensa port,
so define NO_IRQ as (-1).  And use it to override the 8250 driver
hack that compares interrupt numbers with 0 instead of NO_IRQ.

Signed-off-by: Piet Delaney <piet@tensilica.com>
Signed-off-by: Marc Gauthier <marc@tensilica.com>
Signed-off-by: Chris Zankel <chris@zankel.net>