irqchip: spear_shirq: Fix interrupt offset

The ras3 block on spear320 claims to have 3 interrupts. In fact it has
one and 6 reserved interrupts. Account the 6 reserved to this block so
it has 7 interrupts total. That matches the datasheet and the device
tree entries.

Broken since commit 80515a5a

(ARM: SPEAr3xx: shirq: simplify and move
the shared irq multiplexor to DT). Testing is overrated....

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20140619212712.872379208@linutronix.de
Fixes: 80515a5a

 ('ARM: SPEAr3xx: shirq: simplify and move the shared irq multiplexor to DT')
Cc: <stable@vger.kernel.org> # v3.8+
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>