[ion] Remove volatile qualifier from millis return type

Qualification of an integer return type as volatile is ignored by the
compiler. To clarify, an integer value (not a variable), which is what
the return type is, can not be assigned to anyway, so qualification as
volatile does not make sense.

ion/include/ion/timing.h

@@ -11,7 +11,7 @@ void msleep(uint32_t ms);
 
 /* millis is the number of milliseconds ellapsed since a random epoch.
  * On the device, epoch is the boot time. */
-volatile uint64_t millis();
+uint64_t millis();
 
 }
 }

ion/src/device/shared/drivers/timing.cpp

@@ -29,7 +29,7 @@ void usleep(uint32_t us) {
   }
 }
 
-volatile uint64_t millis() {
+uint64_t millis() {
   return MillisElapsed;
 }
 

ion/src/shared/timing.cpp

@@ -3,7 +3,7 @@
 
 static auto start = std::chrono::steady_clock::now();
 
-volatile uint64_t Ion::Timing::millis() {
+uint64_t Ion::Timing::millis() {
   auto elapsed = std::chrono::steady_clock::now() - start;
   return std::chrono::duration_cast<std::chrono::milliseconds>(elapsed).count();
 }