smsc75xx: fix jumbo frame support

smsc75xx: fix jumbo frame support

[ Upstream commit 4c51e53689569398d656e631c17308d9b8e84650 ]

This patch enables RX of jumbo frames for LAN7500.

Previously the driver would transmit jumbo frames succesfully but
would drop received jumbo frames (incrementing the interface errors
count).

With this patch applied the device can succesfully receive jumbo
frames up to MTU 9000 (9014 bytes on the wire including ethernet
header).

Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>