net: qualcomm: rmnet: Use dowlink marker for GRO flushing

Switch to using backlog NAPI instead of GRO cells.
This helps to reduce some latency in the packet processing.

CRs-Fixed: 2285862
Change-Id: Ibd0e9d552c246025c229a8b1100e667022d085a1
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>