genirq/PM: Unexport {suspend,resume}_device_irqs()

Ever since {suspend,resume}_device_irqs() were introduced in 2009
by commit 0a0c5168

 ("PM: Introduce functions for suspending and
resuming device interrupts"), they've been exported even though there
are no module users and never will be:  The functions are solely called
by the PM core, which is always built-in.  Unexport them.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/fad9b50609f9d9828ea14772dbd4d195713f1c4b.1654846687.git.lukas@wunner.de