net: cdc_ncm: do not start timer on an empty skb

We can end up with a freshly allocated tx_curr_skb with no frames
in it.  In this case it does not make any sense to start the timer.
This avoids the timer periodically trying to start tx when there
is nothing in the queue.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>