usb_dispatch.cpp: Lower severity of logging statement.

The first time you run adb, it starts the adbd server and annoyingly
generates the following message:

adb I 02-01 11:57:58 99717 99717 usb_dispatch.cpp:30] using native
backend

Drop the severity of the message to avoid visible spammy messages only
intended for debugging.

Test: code compiles and no more log message in normal adb use
Change-Id: I882092a6cd14bc67980d88416397d51ab8393b1b
diff --git a/adb/client/usb_dispatch.cpp b/adb/client/usb_dispatch.cpp
index 597e66e..f02dccf 100644
--- a/adb/client/usb_dispatch.cpp
+++ b/adb/client/usb_dispatch.cpp
@@ -24,10 +24,10 @@
 
 void usb_init() {
     if (should_use_libusb()) {
-        LOG(INFO) << "using libusb backend";
+        LOG(DEBUG) << "using libusb backend";
         libusb::usb_init();
     } else {
-        LOG(INFO) << "using native backend";
+        LOG(DEBUG) << "using native backend";
         native::usb_init();
     }
 }