net: cdc_ncm: Fix TX zero padding

The zero padding that is added to NTB's does
not zero the memory correctly.
This is because the skb_put modifies the value
of skb_out->len which results in the memset
command not setting any memory to zero as
(ctx->tx_max - skb_out->len) == 0.

I have resolved this by storing the size of
the memory to be zeroed before the skb_put
and using this in the memset call.

Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Reviewed-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>