s390/qeth: reset layer2 attribute on layer switch

After the subdriver's remove() routine has completed, the card's layer
mode is undetermined again. Reflect this in the layer2 field.

If qeth_dev_layer2_store() hits an error after remove() was called, the
card _always_ requires a setup(), even if the previous layer mode is
requested again.
But qeth_dev_layer2_store() bails out early if the requested layer mode
still matches the current one. So unless we reset the layer2 field,
re-probing the card back to its previous mode is currently not possible.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>