HAL_ChibiOS: use uint32_t for bit timing in passthru

we should not use systime_t as it may be 16 bit

libraries/AP_HAL_ChibiOS/RCOutput.cpp

@@ -1565,7 +1565,7 @@ bool RCOutput::serial_write_bytes(const uint8_t *bytes, uint16_t len)
  */
 void RCOutput::serial_bit_irq(void)
 {
-    systime_t now = AP_HAL::micros();
+    uint32_t now = AP_HAL::micros();
     uint8_t bit = palReadLine(irq.line);
     bool send_signal = false;
 

libraries/AP_HAL_ChibiOS/RCOutput.h

@@ -350,7 +350,7 @@ private:
         ioline_t line;
 
         // time the current byte started
-        systime_t byte_start_tick;
+        uint32_t byte_start_tick;
 
         // number of bits we have read in this byte
         uint8_t nbits;
@@ -361,8 +361,8 @@ private:
         // value of completed byte (includes start and stop bits)
         uint16_t byteval;
 
-        // expected time per bit in system ticks
-        systime_t bit_time_tick;
+        // expected time per bit in micros
+        uint32_t bit_time_tick;
 
         // the bit value of the last bit received
         uint8_t last_bit;