s390/qeth: use a static Output Queue array

qeth dynamically allocates an array for storing pointers to its
Output Queue structures. Switch this to a static array - we are
currently limited to 4 Output Queues, so shrinking the qeth_qdio_info
struct by just a few bytes doesn't justify the additional complexity.

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