mlxsw: minimal: Initialize base_mac

Currently base_mac is not initialized which causes wrong reporting of
zeroed parent_id to userspace. Fix this by initializing base_mac
properly.

Fixes: c100e47c

 ("mlxsw: minimal: Add ethtool support")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>