fw: drop RTC I2C frequency to 100 kHz

fw/rtc.c

@@ -50,10 +50,7 @@ void read_rtc(void)
     uint8_t rtc_regs[RTC_REGS];
     int i;
     
-    i2c_set_speed(400);
-    
-    /* Synchronize I2C */
-    i2c_send(0xff, I2C_P);
+    i2c_set_speed(100);
     
     i2c_send(RTC_WCMD, 0);