stmhal: Increase the priority of UART IRQ.

The UARTs have no FIFOs, so if interrupts are disabled
for more than a character time (10 usec at 1 Mbit/sec)
then characters get dropped.

The overhead for handling a UART ISR is about 0.5 usec,
so even at baud rates of 1 Mbit/sec this only corresponds
to about 5% of the CPU. Lower baud rates will have less
of an impact.

stmhal/irq.h

@@ -64,9 +64,14 @@ MP_DECLARE_CONST_FUN_OBJ(pyb_enable_irq_obj);
 //#def  IRQ_PRI_SYSTICK         0
 //#def  IRQ_SUBPRI_SYSTICK                  0
 
+// The UARTs have no FIFOs, so if they don't get serviced quickly then characters
+// get dropped. The handling for each character only consumes about 0.5 usec
+#define IRQ_PRI_UART            1
+#define IRQ_SUBPRI_UART                     0
+
 // Flash IRQ must be higher priority than interrupts of all those components
 // that rely on the flash storage.
-#define IRQ_PRI_FLASH           1
+#define IRQ_PRI_FLASH           2
 #define IRQ_SUBPRI_FLASH                    0
 
 // SDIO must be higher priority than DMA for SDIO DMA transfers to work.
@@ -93,9 +98,6 @@ MP_DECLARE_CONST_FUN_OBJ(pyb_enable_irq_obj);
 #define IRQ_PRI_CAN             7
 #define IRQ_SUBPRI_CAN                      0
 
-#define IRQ_PRI_UART            13
-#define IRQ_SUBPRI_UART                     0
-
 // Interrupt priority for non-special timers.
 #define IRQ_PRI_TIMX            14
 #define IRQ_SUBPRI_TIMX                     0