litex_liteeth: Fix a double free in the remove function

'netdev' is a managed resource allocated in the probe using
'devm_alloc_etherdev()'.
It must not be freed explicitly in the remove function.

Fixes: ee7da21a ("net: Add driver for LiteX's LiteETH network interface")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>