stmhal/irq: Shift IRQ priorities of TIM and EXTINT to be above PENDSV.

This way, Timer and ExtInt callbacks can interrupt the low-priority
pendsv handler (for example thread switching).

stmhal/irq.h

@@ -133,10 +133,10 @@ MP_DECLARE_CONST_FUN_OBJ_0(pyb_irq_stats_obj);
 #define IRQ_SUBPRI_CAN                      0
 
 // Interrupt priority for non-special timers.
-#define IRQ_PRI_TIMX            14
+#define IRQ_PRI_TIMX            13
 #define IRQ_SUBPRI_TIMX                     0
 
-#define IRQ_PRI_EXTINT          15
+#define IRQ_PRI_EXTINT          14
 #define IRQ_SUBPRI_EXTINT                   0
 
 // PENDSV should be at the lowst priority so that other interrupts complete