net: rmnet_data: Add counters for downlink checksum offload return codes

Added counter array to /sys/module/rmnet_data/parameters/checksum_dl_stats
for non-realtime analysis of checksum offload.

Change-Id: I749c09147325fd0f871c34ff17e2546b68898faa
Signed-off-by: Harout Hedeshian <harouth@codeaurora.org>