libertas: Move SET_BOOT2_VER command to if_usb where it belongs

This is meaningless for non-USB devices and unimplemented in their
firmware. It's somewhat dubious for USB devices too, but that's a
different story.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>