fastboot: gracefully handle failure to open a USB device on OS X.

High Sierra restricts opening some USB devices (e.g. the touchbar)
to processes that have specific entitlements. Ignore devices that we
can't open.

Bug: http://b/64292422
Test: manual
Change-Id: I6074b53a365b8d936610bafea60244f8bba1a33f
diff --git a/fastboot/usb_osx.cpp b/fastboot/usb_osx.cpp
index 9069baa..e95b049 100644
--- a/fastboot/usb_osx.cpp
+++ b/fastboot/usb_osx.cpp
@@ -93,12 +93,9 @@
     SInt32 score;
     UInt8 interfaceNumEndpoints;
 
-    // Placing the constant KIOUSBFindInterfaceDontCare into the following
-    // fields of the IOUSBFindInterfaceRequest structure will allow us to
-    // find all of the interfaces
-    request.bInterfaceClass = kIOUSBFindInterfaceDontCare;
-    request.bInterfaceSubClass = kIOUSBFindInterfaceDontCare;
-    request.bInterfaceProtocol = kIOUSBFindInterfaceDontCare;
+    request.bInterfaceClass = 0xff;
+    request.bInterfaceSubClass = 0x42;
+    request.bInterfaceProtocol = 0x03;
     request.bAlternateSetting = kIOUSBFindInterfaceDontCare;
 
     // Get an iterator for the interfaces on the device
@@ -282,7 +279,6 @@
             &plugin, &score);
 
     if ((kr != 0) || (plugin == NULL)) {
-        ERR("Unable to create a plug-in (%08x)\n", kr);
         goto error;
     }
 
@@ -436,8 +432,7 @@
 
         if (try_device(device, &h) != 0) {
             IOObjectRelease(device);
-            ret = -1;
-            break;
+            continue;
         }
 
         if (h.success) {