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: b06eb0173ef1 ("irqchip: Add TB10x interrupt controller driver")
Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Christian Ruppert <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-tb10x.c

···
 		break;
 	case IRQ_TYPE_NONE:
 		flow_type = IRQ_TYPE_LEVEL_LOW;
+		fallthrough;
 	case IRQ_TYPE_LEVEL_LOW:
 		mod ^= im;
 		pol ^= im;