ethernet: 8390: remove unnecessary check of mem

The 'mem' returned by platform_get_resource() has been checked in probe
function, so it is no need do this check in remove function.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220927151406.797800-1-yangyingliang@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>