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: NAdrian McMenamin <adrian@newgolddream.dyndns.info>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>