USBNET: increase max rx/tx qlen for improving USB3 thoughtput

The default RX_QLEN()/TX_QLEN() didn't consider super speed
USB device, so only max 4 URBs are scheduled at the same time
for tx/rx, then USB3 NIC can't perform very well.

With this patch, both rx and tx thoughput are increased more than
100Mbps when doing iperf test on ax88179_178a USB 3.0 NIC.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>