network: move set-MAC and set-nomaster operations out of link_up()

network: move set-MAC and set-nomaster operations out of link_up()

These should not be bundled into the link_up() operation, as that is
not (currently) called during interface configuration if the interface
already is IFF_UP, which is unrelated to the need to change the mac
to a user-defined value, or set 'nomaster' on the interface.

Additionally, there is no need to re-set the mac or re-assert nomaster
every time the interface is brought up; those should be only part of
normal initial interface configuration.

Fixes: #17391