net: ethernet: mtk_eth_soc: delete some dead code

The debugfs_create_dir() function never returns NULL.  It does return
error pointers but in normal situations like this there is no need to
check for errors.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20211217071037.GE26548@kili


Signed-off-by: Jakub Kicinski <kuba@kernel.org>