Revert "irqchip/imx-irqsteer: Constify irq_chip struct"

This reverts commit 06a93b72 which is
commit e9a50f12 upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Ib907b82915f576c228c2b293b48ee0a9a58e9d82
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>