w5100: using eth_hw_addr_random() for random MAC and set device flag

Using eth_hw_addr_random() to generate a random Ethernet address
(MAC) to be used by a net device and set addr_assign_type.
Not need to duplicating its implementation.

spatch with a semantic match is used to found this problem.
(http://coccinelle.lip6.fr/

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>