fastboot device: Disallow implicit conversion from unique_fd to int.

Do not use the implicit cast from unique_fd to int so
that it is clearer to the reader what the ownership model
is.

Test: pass
Change-Id: I66563eb2bd06f6a712a5afd4c6009f9b25a55de6
diff --git a/fastboot/device/usb.cpp b/fastboot/device/usb.cpp
index 4bee7b2..4115a6d 100644
--- a/fastboot/device/usb.cpp
+++ b/fastboot/device/usb.cpp
@@ -82,7 +82,7 @@
     int orig_len = len;
     while (len > 0) {
         int write_len = std::min(USB_FFS_BULK_SIZE, len);
-        int n = write(h->bulk_in, buf, write_len);
+        int n = write(h->bulk_in.get(), buf, write_len);
         if (n < 0) {
             D("ERROR: fd = %d, n = %d: %s", h->bulk_in.get(), n, strerror(errno));
             return -1;
@@ -103,7 +103,7 @@
     unsigned count = 0;
     while (len > 0) {
         int read_len = std::min(USB_FFS_BULK_SIZE, len);
-        int n = read(h->bulk_out, buf, read_len);
+        int n = read(h->bulk_out.get(), buf, read_len);
         if (n < 0) {
             D("ERROR: fd = %d, n = %d: %s", h->bulk_out.get(), n, strerror(errno));
             return -1;