powerpc/mpc5121_ads_cpld: Remove use of NO_IRQ_IGNORE

As NO_IRQ_IGNORE is only used between the static function cpld_pic_get_irq
and its caller cpld_pic_cascade, and cpld_pic_cascade only uses it to
suppress calling handle_generic_irq, we can change these uses to NO_IRQ
and remove the extra tests and pathlength in cpld_pic_cascade.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>