net: qualcomm: rmnet: Do not zero out transport checksum

Hardware does not use the value populated in the transport checksum
field for checksum computation in MAPv5 format if checksum required
bit is set. The partial checksum can be left as is in these cases.

Explicitly marking the field as 0 could potentially cause incorrect
verdicts in hardware in translation scenarios for UDPv4.

CRs-fixed: 2594257
Change-Id: I8d9e38e6af0df344ceebd630a895b1d7f467b1db
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>