staging: ft1000: rename struct ft1000_device to ft1000_usb

After unifying struct ft1000_info, struct ft1000_device is now wrong name.
Rename it to ft1000_usb to match its semantics.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>