pru: Avoid calling "slp" instruction on pru0

If pru0 waits in sleep mode then a full beaglebone reboot is needed to
reprogram the pru0 firmware.  For now, avoid using the "slp"
instruction as a workaround.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/pru/pru0.c

@@ -166,7 +166,9 @@ process_io(void)
         int can_sleep = check_can_read();
         if (can_sleep) {
             flush_messages();
-            asm("slp 1");
+            while (!(read_r31() & (1 << (WAKE_PRU0_IRQ + R31_IRQ_OFFSET)))) {
+                //asm("slp 1");
+            }
         }
     }
 }