net: qualcomm: rmnet: Handle command packets with checksum trailer

When using the MAPv4 packet format in conjunction with MAP commands,
a dummy DL checksum trailer will be appended to the packet. Before
this packet is sent out as an ACK, the DL checksum trailer needs to be
removed.

CRs-Fixed: 2156182
Change-Id: I6d97519a81692e86655abe85fce287ead075a40e
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>