[ext] update libopencm3

Reason is just to get the fix for usb control callbacks so luftboot works again.
Since rcc_ppre1_frequency and rcc_ppre2_frequency were also renamed in the meantime, make appropriate changes.

sw/airborne/arch/stm32/mcu_arch.c

@@ -127,7 +127,7 @@ static inline uint32_t rcc_get_ppre2(void)
 uint32_t timer_get_frequency(uint32_t timer_peripheral)
 {
   switch (timer_peripheral) {
-      // Timers on APB1
+      // Timers on high speed APB2
     case TIM1:
     case TIM8:
 #ifdef TIM9
@@ -140,14 +140,15 @@ uint32_t timer_get_frequency(uint32_t timer_peripheral)
     case TIM11:
 #endif
       if (!rcc_get_ppre2())
-        // no APB2 prescaler
       {
-        return rcc_ppre2_frequency;
+        /* without APB2 prescaler, runs at APB2 freq */
+        return rcc_apb2_frequency;
       } else {
-        return rcc_ppre2_frequency * 2;
+        /* with any ABP2 prescaler, runs at 2 * APB2 freq */
+        return rcc_apb2_frequency * 2;
       }
 
-      // timers on APB2
+      // timers on low speed APB1
     case TIM2:
     case TIM3:
     case TIM4:
@@ -164,11 +165,12 @@ uint32_t timer_get_frequency(uint32_t timer_peripheral)
     case TIM14:
 #endif
       if (!rcc_get_ppre1())
-        // no APB2 prescaler
       {
-        return rcc_ppre1_frequency;
+        /* without APB1 prescaler, runs at APB1 freq */
+        return rcc_apb1_frequency;
       } else {
-        return rcc_ppre1_frequency * 2;
+        /* with any ABP1 prescaler, runs at 2 * APB1 freq */
+        return rcc_apb1_frequency * 2;
       }
     default:
       // other timers currently not supported

sw/airborne/arch/stm32/mcu_periph/i2c_arch.c

@@ -1172,10 +1172,10 @@ void i2c_setbitrate(struct i2c_periph *periph, int bitrate)
       bitrate = 3000;
     }
 
-    // rcc_ppre1_frequency is normally configured to max: 36MHz on F1 and 42MHz on F4
+    // rcc_apb1_frequency is normally configured to max: 36MHz on F1 and 42MHz on F4
     // in fast mode: 2counts low 1 count high -> / 3:
     // in standard mode: 1 count low, 1 count high -> /2:
-    devider = (rcc_ppre1_frequency / 2000) / (bitrate / 1000);
+    devider = (rcc_apb1_frequency / 2000) / (bitrate / 1000);
 
     // never allow faster than 600kbps
     if (devider < 20) {