irqchip: crossbar: Introduce centralized check for crossbar write

This is a basic check to ensure that crossbar register needs to be
written. This ensures that we have a common check which is used in
both map and unmap logic.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Link: https://lkml.kernel.org/r/1403766634-18543-15-git-send-email-r.sricharan@ti.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>