net: qualcomm: rmnet: Remove redundant check when stamping map header

We already check the headroom once in rmnet_map_egress_handler(),
so this is not needed.

Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>