genirq/debugfs: Reset domain debugfs_file on removal of the debugfs file

When removing a debugfs file for a given irq domain, we fail to clear the
corresponding field, meaning that the corresponding domain won't be created
again if we need to do so.

It turns out that this is exactly what irq_domain_update_bus_token does
(delete old file, update domain name, recreate file).

This doesn't have any impact other than making debug more difficult, but we
do value ease of debugging... So clear the debugfs_file field.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20181001100522.180054-2-marc.zyngier@arm.com