staging/ft1000-usb: fix problems found by sparse

In the original code, address space annotations are missing,
which hides a possible unchecked user pointer access.

Two functions use a lot of stack space.

Extern declarations are all in the wrong place, which leads
to type differences between caller and callee in some cases.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>