s390/qeth: use all configured RX buffers

The (misplaced) comment doesn't make any sense, enforcing an
uninitialized RX buffer won't help with IRQ reduction.

So make the best use of all available RX buffers.

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