staging: mt7621-eth: Fix memory leak in mtk_add_mac() error path

Fix memory leak in error path of mtk_add_mac() by make sure to free
the allocated netdev.

Fixes: e3cbf478 ('staging: mt7621-eth: add the drivers core files')
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>