pru: Avoid using a timer wake time of exactly zero

It appears the PRU timer will not trigger on a match of exactly zero.
If the next timer happened to be set to exactly zero, it could cause
software timers to get stuck and result in "move queue empty" errors.

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

src/pru/main.c

@@ -56,6 +56,8 @@ irq_wait(void)
 static void
 timer_set(uint32_t value)
 {
+    if (!value)
+        value = 1;
     CT_IEP.TMR_CMP0 = value;
 }