s390/qeth: remove duplicated carrier state tracking

The netdevice is always available, apply any carrier state changes to it
without caching them.
On a STARTLAN event (ie. carrier-up), defer updating the state to
qeth_core_hardsetup_card() in the subsequent recovery action.

Also remove the carrier-state checks from the xmit routines. Stopping
transmission on carrier-down is the responsibility of upper-level code
(eg see dev_direct_xmit()).

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