genirq: fix reference in devm_request_threaded_irq comment

It should be request_threaded_irq, not request_irq

[jkosina@suse.cz: not that it would matter, as both have the same
 set of arguments anyway, but for sake of consistency ...]

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>