rm48x50: rt_interrupt_nest should be `volatile rt_uint8_t`

libcpu/arm/cortex-r4/interrupt.c

@@ -22,7 +22,7 @@
 /* exception and interrupt handler table */
 struct rt_irq_desc irq_desc[MAX_HANDLERS];
 
-extern rt_uint32_t rt_interrupt_nest;
+extern volatile rt_uint8_t rt_interrupt_nest;
 
 /* exception and interrupt handler table */
 rt_uint32_t rt_interrupt_from_thread, rt_interrupt_to_thread;