net: eth-uclass: avoid running start() twice without stop()

Running the start() handler twice without a stop() inbetween completely
breaks communication for some ethernet drivers like fec_mxc.

eth_halt() is called before each eth_init(). Due to the switch to
eth_is_active() in commit 68acb51f ("net: Only call halt on a driver
that has been init'ed"), this is not sufficient anymore when netconsole
is active: eth_init_state_only()/eth_halt_state_only() manipulate the
state check that is performed by eth_is_active() without actually
calling into the driver.

The issue can be triggered by starting a network operation (e.g. ping or
tftp) while netconsole is active.

Add an additional "running" flag that reflects the actual state of the
driver and use it to ensure that eth_halt() actually stops the device as
it is supposed to.

Fixes: 68acb51f ("net: Only call halt on a driver that has been init'ed")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>