lan78xx: Fix failure in USB Full Speed

Fix initialize the uninitialized tx_qlen to an appropriate value when USB
Full Speed is used.

Fixes: 55d7de9d

 ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Yuiko Oshino <yuiko.oshino@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>