usb: gadget: udc: Avoid tasklet passing a global

There's no reason for the tasklet callback to set an argument since it
always uses a global. Instead, use the global directly, in preparation
for converting the tasklet subsystem to modern callback conventions.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kees Cook <keescook@chromium.org>