net: eth-uclass: guard against reentrant eth_init()/eth_halt() calls

With netconsole, any log message can result in an eth_init(), possibly
causing an reentrant call into eth_init() if a driver's ops print
anything:

    eth_init() -> driver.start() -> printf() -> netconsole -> eth_init()
    eth_halt() -> driver.stop() -> printf() -> netconsole -> eth_init()

Rather than expecting every single Ethernet driver to handle this case,
prevent the reentrant calls in eth_init() and eth_halt().

The issue was noticed on an AM62x board, where a bootm after
simultaneous netconsole and TFTP would result in a crash.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Link: https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1350550/sk-am62a-lp-am65_cpsw_nuss_port-error-in-u-boot-while-using-netconsole-functionality/