fix compiler complaints about volatile on macos

firmware/mock_hardware/common/ftdi.c

@@ -17,13 +17,13 @@ u8* ftdi_rx_buf(void)
     return 0;
 }
 
-u8 ftdi_rx_bytes(void)
+volatile u8 ftdi_rx_bytes(void)
 {
     return 0;
 }
 
-u8 ftdi_rx_busy(void) { return 0; }
-u8 ftdi_tx_busy(void) { return 0; }
+volatile u8 ftdi_rx_busy(void) { return 0; }
+volatile u8 ftdi_tx_busy(void) { return 0; }
 
 u8 ftdi_connected(void)
 {

firmware/mock_hardware/include/ftdi.h

@@ -13,7 +13,7 @@ void ftdi_change(uhc_device_t* dev, u8 plug);
 void ftdi_setup(void);
 
 u8* ftdi_rx_buf(void);
-u8 ftdi_rx_bytes(void);
-u8 ftdi_rx_busy(void);
-u8 ftdi_tx_busy(void);
+volatile u8 ftdi_rx_bytes(void);
+volatile u8 ftdi_rx_busy(void);
+volatile u8 ftdi_tx_busy(void);
 u8 ftdi_connected(void);

firmware/mock_hardware/mock_serial.c

@@ -72,7 +72,7 @@ u8* mock_serial_rx_buf(void)
     return mock_serial_current_message_buffer;
 }
 
-u8 mock_serial_rx_bytes(void)
+volatile u8 mock_serial_rx_bytes(void)
 {
     return mock_serial_current_message_length;
 }