net: cdc_ether: fix divide by 0 on bad descriptors

net: cdc_ether: fix divide by 0 on bad descriptors

commit 2cb80187ba065d7decad7c6614e35e07aec8a974 upstream.

Setting dev->hard_mtu to 0 will cause a divide error in
usbnet_probe. Protect against devices with bogus CDC Ethernet
functional descriptors by ignoring a zero wMaxSegmentSize.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Acked-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: parsing code is organised differently]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>