net: rmnet_data: Fix QMAP ACK's 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 takes this into account for data
packets but not for command packets. As a result, the additional
8 bytes were sent to hardware as part of the MAP ACK's and were
subsequently dropped.

Fix this by truncating the extra bytes of the DL checksum trailer from
the MAP ACK.

CRs-Fixed: 961336
Change-Id: I175dde695e7ee09d16c99fb71898d635c7a812ab
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>