lan78xx: Fix failure in USB Full Speed

[ Upstream commit a5b1379afbfabf91e3a689e82ac619a7157336b3 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>