Allocate a buffer with kmalloc for receiving the parameters string
descriptor with usb_control_msg, instead of using a buffer on the stack,
as the latter is unsafe. Use an enum for indices into the buffer to
ensure the buffer size if sufficient.

This fixes the static checker error "doing dma on the stack (buf)".
Signed-off-by: NNikolai Kondrashov <spbnick@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>