genirq/generic_chip: Constify irq_generic_chip_ops

The only usage of irq_generic_chip_ops is to pass its address to
irq_domain_add_linear() which takes a pointer to const struct
irq_domain_ops. Make it const to allow the compiler to put it in
read-only memory.

[ tglx: Fixed subject prefix ]

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20211130214043.1257585-1-rikard.falkeborn@gmail.com