The code was using a homegrown method of looking up the offset
from the irq domain, not to be encouraged. Use the proper
irq_find_mapping() call instead.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>