usb: usbtest: Convert timers to use timer_setup()

In preparation for unconditionally passing the struct timer_list pointer to
all timer callbacks, switch to using the new timer_setup() and from_timer()
to pass the timer pointer explicitly. Also adds missing call to
destroy_timer_on_stack();

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>