AS3525v1/2: Enable nested handling of interrupts

Mostly for the sake of reducing latency for audio servicing where other service
routines can take a long time to complete, leading to occasional drops of a
few samples, especially in recording, where they are fairly frequent.

One mystery that remains is GPIOA IRQ being interrupted causes strange
undefined instruction exceptions, most easily produced on my Fuze V2 with a
scrollwheel. Making GPIOA the top ISR for now, thus not interruptible, cures it.

SVC mode is used during the actual calls. Hopefully the SVC stack size is
sufficient. Prologue and epilogue code only uses the IRQ stack and is large
enough.

Any routine code that should not be interrupted should disable IRQ itself from
here on in.


git-svn-id: svn://svn.rockbox.org/rockbox/trunk@31642 a1c6a512-1295-4272-9138-f99709370657

firmware/target/arm/as3525/fmradio-i2c-as3525.c

@@ -195,8 +195,8 @@ void tuner_isr(void)
     /* read and clear the interrupt */
     if (GPIOA_MIS & (1<<4)) {
         semaphore_release(&rds_sema);
+        GPIOA_IC = (1<<4);
     }
-    GPIOA_IC = (1<<4);
 }
 
 /* Captures RDS data and processes it */