stdio: Remove redundant code around stdio_register() calls

stdio: Remove redundant code around stdio_register() calls

There is no point in setting a structure's memory to NULL when it has
already been zeroed with memset().

Also, there is no need to create a stub function for stdio to call - if the
function is NULL it will not be called.

This is a clean-up, with no change in functionality.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>