s390/qeth: convert layer attribute to enum

While the raw values are fixed due to their use in a sysfs attribute,
we can still use the proper QETH_DISCIPLINE_* enum within the driver.

Also move the initialization into qeth_set_initial_options(), along with
all other user-configurable fields.

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