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.

CRs-Fixed: 2156182
Change-Id: Ic19cf2a1183c4aceb5174d6e77f8fd73152b6cd2
Fixes: ceed73a2 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>