genirq/manage: Add buslock back in to enable_irq()

The locking was changed from a buslock to a plain lock, but the patch
description states there was no functional change. Assuming this was
accidental so reverting to using the buslock.

Fixes: bddd10c5 ("genirq/manage: Rework enable_irq()")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20251023154901.1333755-4-ckeepax@opensource.cirrus.com