irqchip/tb10x: Use 'fallthrough' to eliminate a warning

Use the 'fallthrough' macro to document that this switch case
does indeed fall through to the next case.

../drivers/irqchip/irq-tb10x.c: In function 'tb10x_irq_set_type':
../drivers/irqchip/irq-tb10x.c:62:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   62 |   flow_type = IRQ_TYPE_LEVEL_LOW;
../drivers/irqchip/irq-tb10x.c:63:2: note: here
   63 |  case IRQ_TYPE_LEVEL_LOW:
      |  ^~~~

Fixes: b06eb017 ("irqchip: Add TB10x interrupt controller driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Christian Ruppert <christian.ruppert@abilis.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210422051620.23021-1-rdunlap@infradead.org

drivers/irqchip/irq-tb10x.c

@@ -60,6 +60,7 @@ static int tb10x_irq_set_type(struct irq_data *data, unsigned int flow_type)
 		break;
 	case IRQ_TYPE_NONE:
 		flow_type = IRQ_TYPE_LEVEL_LOW;
+		fallthrough;
 	case IRQ_TYPE_LEVEL_LOW:
 		mod ^= im;
 		pol ^= im;