usb: Suggest LIBUSB_DEBUG=3 for troubleshooting

LIBUSB_DEBUG=4 is too verbose and mostly useless.

Author: xlz

README.md

@@ -76,7 +76,7 @@ Windows 7 does not have great USB 3.0 drivers. In our testing the above known wo
 
 Disabling USB selective suspend/autosuspend might be helpful to ameliorate transfer problems.
 
-When you report USB issues, please attach relevant debug log from running the program with environment variable `LIBUSB_DEBUG=4`, and relevant log from `dmesg`.
+When you report USB issues, please attach relevant debug log from running the program with environment variable `LIBUSB_DEBUG=3`, and relevant log from `dmesg`.
 
 ### I'm seeing the color camera stream, but no depth/IR (black windows).
 

src/transfer_pool.cpp

@@ -108,7 +108,7 @@ void TransferPool::submit(size_t num_parallel_transfers)
   }
 
   if (failcount == num_parallel_transfers)
-    LOG_ERROR << "all submissions failed. Try debugging with environment variable: LIBUSB_DEBUG=4.";
+    LOG_ERROR << "all submissions failed. Try debugging with environment variable: LIBUSB_DEBUG=3.";
 }
 
 void TransferPool::cancel()

src/usb_control.cpp

@@ -190,7 +190,7 @@ UsbControl::~UsbControl()
 }
 
 #define CHECK_LIBUSB_RESULT(__CODE, __RESULT) if((__CODE = (__RESULT == LIBUSB_SUCCESS ? Success : Error)) == Error) LOG_ERROR
-#define WRITE_LIBUSB_ERROR(__RESULT) libusb_error_name(__RESULT) << " " << libusb_strerror((libusb_error)__RESULT) << ". Try debugging with environment variable: export LIBUSB_DEBUG=4 ."
+#define WRITE_LIBUSB_ERROR(__RESULT) libusb_error_name(__RESULT) << " " << libusb_strerror((libusb_error)__RESULT) << ". Try debugging with environment variable: export LIBUSB_DEBUG=3 ."
 
 UsbControl::ResultCode UsbControl::setConfiguration()
 {