USB: serial: io_edgeport: drop unneeded forward declarations

Forward declarations make the code larger and rewrites harder. Harder as
they are often omitted from global changes. Remove forward declarations
which are not really needed, i.e. the definition of the function is
before its first use.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ johan: update the prototype comments ]
Signed-off-by: Johan Hovold <johan@kernel.org>