drivers: Add access annotations to USB helpers

Mostly for completeness sake, doing this did not find any errors (but
might catch some issues with fixed buffer lengths).

libfprint/drivers/aesx660.c

@@ -54,6 +54,7 @@ static void complete_deactivation (FpImageDevice *dev);
 #define CALIBRATE_DATA_LEN 4
 #define FINGER_DET_DATA_LEN 4
 
+FP_GNUC_ACCESS (read_only, 3, 4)
 static void
 aesX660_send_cmd_timeout (FpiSsm                *ssm,
                           FpDevice              *_dev,
@@ -70,6 +71,7 @@ aesX660_send_cmd_timeout (FpiSsm                *ssm,
   fpi_usb_transfer_submit (transfer, timeout, NULL, callback, NULL);
 }
 
+FP_GNUC_ACCESS (read_only, 3, 4)
 static void
 aesX660_send_cmd (FpiSsm                *ssm,
                   FpDevice              *dev,

libfprint/drivers/upektc_img.c

@@ -78,6 +78,7 @@ upektc_img_cmd_update_crc (unsigned char *cmd_buf, size_t size)
   cmd_buf[size - 1] = (crc & 0xff00) >> 8;
 }
 
+FP_GNUC_ACCESS (read_only, 3, 4)
 static void
 upektc_img_submit_req (FpiSsm                *ssm,
                        FpImageDevice         *dev,

libfprint/drivers/vfs0050.c

@@ -42,6 +42,7 @@ async_write_callback (FpiUsbTransfer *transfer, FpDevice *device,
 }
 
 /* Send data to EP1, the only out endpoint */
+FP_GNUC_ACCESS (read_only, 3, 4)
 static void
 async_write (FpiSsm   *ssm,
              FpDevice *dev,

libfprint/drivers/vfs301_proto.c

@@ -93,6 +93,7 @@ usb_recv (FpDeviceVfs301 *dev, guint8 endpoint, int max_bytes, FpiUsbTransfer **
     *out = g_steal_pointer (&transfer);
 }
 
+FP_GNUC_ACCESS (read_only, 2, 3)
 static void
 usb_send (FpDeviceVfs301 *dev, const guint8 *data, gssize length, GError **error)
 {