usb: misc: Add support for test modes via external hub

Some external hubs cannot enter into test mode if EHSET fixture
is connected to one of its downstream ports. Add support to
enable test modes on any desired downstream port of an external
hub. The port and the test mode can be selected via sysfs nodes.

For example, to send TEST_J packet on downstream port 4 of
external hub 3-1:1.0:
echo 4 > /sys/bus/usb/devices/3-1/3-1\:1.0/test_j_portnum

Change-Id: I145f150bfe8557844cb150cf39fafe03a18bd955
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>