memory: tegra: Delete some dead code

The debugfs_create_dir() function never returns NULL and anyway the
correct behavior is to ignore errors in this situation.  The
debugfs_create_file() will become a no-op if "emc->debugfs.root" is an
error pointer.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>