staging: mt7621-eth: Use eth_hw_addr_random()

Use eth_hw_addr_random() to set a random dev_addr and update
addr_assign_type.

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>