sh: Fix dreamcast build for IRQ changes.

When the irq.h changes went in, the dreamcast code was still
referencing an old value. Switch it back to the IRQ number,
which fixes this:

arch/sh/boards/dreamcast/irq.c: In function `disable_systemasic_irq':
arch/sh/boards/dreamcast/irq.c:59: error: `OFFCHIP_IRQ_BASE' undeclared (first
use in this function)
arch/sh/boards/dreamcast/irq.c:59: error: (Each undeclared identifier is reported only once
arch/sh/boards/dreamcast/irq.c:59: error: for each function it appears in.)

Reported-by: Adrian McMenamin <adrian@newgolddream.dyndns.info>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>