net: natsemi: fix hw address initialization for jazz and xtensa

Use eth_hw_addr_set function instead of writing the address directly to
net_device::dev_addr.

Fixes: adeef3e32146 ("net: constify netdev->dev_addr")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/r/20211130143600.31970-1-jcmvbkbc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>