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