genirq: Don't overwrite interrupt thread flags on setup

Chris reported that the recent affinity management changes result in
overwriting the already initialized thread flags.

Use set_bit() to set the affinity bit instead of assigning the bit value to
the flags.

Fixes: 801afdfbfc ("genirq: Fix interrupt threads affinity vs. cpuset isolated partitions")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://patch.msgid.link/87ecp0e4cf.ffs@tglx
Closes: https://lore.kernel.org/all/20251212014848.3509622-1-clm@meta.com

kernel/irq/manage.c

@@ -1414,7 +1414,7 @@ setup_irq_thread(struct irqaction *new, unsigned int irq, bool secondary)
 	 * Ensure the thread adjusts the affinity once it reaches the
 	 * thread function.
 	 */
-	new->thread_flags = BIT(IRQTF_AFFINITY);
+	set_bit(IRQTF_AFFINITY, &new->thread_flags);
 
 	return 0;
 }