powerpc: 52xx: provide a default in mpc52xx_irqhost_map()

My gcc-4.3.5 fails to compile due to:

|cc1: warnings being treated as errors
|arch/powerpc/platforms/52xx/mpc52xx_pic.c: In function ‘mpc52xx_irqhost_map’:
|arch/powerpc/platforms/52xx/mpc52xx_pic.c:343: error: ‘irqchip’ may be used uninitialized in this function

since commit e34298c9 ("powerpc: 52xx: nop out unsupported critical
IRQs"). This warning is complete crap since only values 0…3 are possible
which are checked but gcc fails to understand that. I wouldn't care much
but since this is compiled with -Werror I made this patch.
While add it, I replaced the warning from l2irq to l1irq since this is
the number that is evaluated.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>