irqchip: Properly fetch the per cpu offset

The raw_cpu_read() conversion dropped the fetch of the offset
from base->percpu_base in gic_get_percpu_base.

Signed-off-by: Christoph Lameter <cl@linux.com>
Reported-and-tested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Tejun Heo <tj@kernel.org>