net: rmnet_data: Fix deaggregation to work with DL checksum offload

Specifications state that the MAP packet length in the MAP header do
not account for the number of bytes of packet trailer from DL checksum
offload. Current implementation does not take this into account when
MAP aggregation is enabled.

Fix this by accounting for the extra bytes of the DL checksum trailer
if DL checksum offload is enabled when computing packet length during
MAP deaggregation.

Change-Id: I9c10bb9726413b1f14f94210dbe194c2c15349f5
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>