net: fix network drivers ndo_start_xmit() return values

Fix up remaining drivers returning a magic or an errno value from their
ndo_start_xmit() functions that were missed in the first pass:

- isdn_net: missed conversion
- bpqether: missed conversion: skb is freed, so return NETDEV_TX_OK
- hp100: intention appears to be to resubmit skb once resources are
  available, but due to no queue handling it is dropped for now.
- lapbether: skb is freed, so return NETDEV_TX_OK

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>