s390/qeth: allow configuration of TX queues for z/VM NICs

Add support for ETHTOOL_SCHANNELS to change the count of active
TX queues.

Since all TX queue structs are pre-allocated and -registered, we just
need to trivially adjust dev->real_num_tx_queues.

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