genirq: Move irq_chip_write_msi_msg() helper to core

No point to expose this to the world. The only legitimate user is the
core code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>

diff --cc include/linux/irq.h
Simple merge

diff --cc kernel/irq/msi.c
Simple merge