cxl: no need to check return value of debugfs_create functions

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Because there's no need to check, also make the return value of the
local debugfs_create_io_x64() call void, as no one ever did anything
with the return value (as they did not need to.)

And make the cxl_debugfs_* calls return void as no one was even checking
their return value at all.

Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>