sh_eth: remove open coded netif_running()

It had a purpose back in the days, but today we have a handy helper.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230321065826.2044-1-wsa+renesas@sang-engineering.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>