net: mvneta: Use prepare/commit API to simplify MAC address setting

Use eth_prepare_mac_addr_change and eth_commit_mac_addr_change, instead
of manually checking and storing the MAC address, which makes the
code slightly more robust. This fixes the lack of valid MAC address check
in the driver's .ndo_set_mac_address hook.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>