usb: usbtest: Always clear halt else further tests will fail

In test_halt() we set an endpoint halt condition and return on halt verification
failure, then the enpoint will remain halted and all further tests related
to that enpoint will fail. This is because we don't tackle endpoint halt error condition
in any of the tests. To avoid that situation, make sure to clear the
halt condition before exiting test_halt().

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>