genirq: Fix editing error in a comment

When the comment was reflowed to a wider format, the "*" snuck in.

Fixes: ae88a23b

 ("irq: refactor and clean up the free_irq() code flow")
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180617124018.25539-1-j.neuschaefer@gmx.net