net: osci_lan: make mac_addr static

This makes the global variable mac_addr static, to prevent conflicts
with other network drivers

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
net: osci_lan: set default to n

Don't build the osci_lan Ethernet driver by default, as it's only required
for the nSIM OSCI model.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>