staging: use of_get_ethdev_address()

Use the new of_get_ethdev_address() helper for the cases
where dev->dev_addr is passed in directly as the destination.

  @@
  expression dev, np;
  @@
  - of_get_mac_address(np, dev->dev_addr)
  + of_get_ethdev_address(np, dev)

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20211026175038.3197397-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>