irq: Fixed debug_uart priority in comment

Due to commit: 43edb28, debug_uart NIVC priority
has been reduced to 0xF for fixing issue #55.

So the comment in irq needs to change to 0xF,
instead of 0x2.

platform/irq.c

@@ -19,10 +19,9 @@ void irq_init(void)
 	NVIC_SetPriority(BusFault_IRQn, 0x1, 0);
 	NVIC_SetPriority(UsageFault_IRQn, 0x1, 0);
 
-	/* Priority 0x2 - debug_uart */
-
 	NVIC_SetPriority(SysTick_IRQn, 0x3, 0);
 
+	/* Priority 0xF - debug_uart */
 	NVIC_SetPriority(SVCall_IRQn, 0xF, 0);
 	NVIC_SetPriority(PendSV_IRQn, 0xF, 0);
 }