stdio: Fix class iteration in stdio_add_devices()

stdio: Fix class iteration in stdio_add_devices()

There is a complaint in the code that iterates keyboards that we don't
have the _check variant of class iterator but we in fact do, use it.

In the code that iterates video devices there is an attempt to print
errors but the simple iterator does not return a device when there is an
error. Use the _check variant of the iterator as well.

Also format error messages consistently.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>