USB: serial: keyspan: drop unneeded forward declarations

Forward declarations make the code larger, harder to follow and rewrite.
Harder as the declarations are often omitted from global changes. Remove
forward declarations which are not really needed, i.e. when 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>
Signed-off-by: Johan Hovold <johan@kernel.org>