net: qualcomm: rmnet: Fix leak on transmit failure

If a skb in transmit path does not have sufficient headroom to add
the map header, the skb is not sent out and is never freed.

Fixes: ceed73a2

 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>