pktgen: set correct max and min in pktgen_setup_inject()

In 882716604ec "pktgen: fix multiple queue warning" we added special
logic to handle the case where ntxq is zero.  It's not clear to me that
ntxq can actually be zero.  But if it were then we would set
->queue_map_min and ->queue_map_max to USHRT_MAX when probably we want
to set them to zero?

Signed-off-by: David S. Miller <davem@davemloft.net>