net: qualcomm: rmnet: Only update headers for coalesced scenarios

For packets using the nonlinear descriptor path, the header length fields
only need to be updated when stamping the GSO information (or when
calculating a bad checksum if the packet was marked as invalid). Only
touch the header fields in these specific scenarios.

Change-Id: I678195507a3db0f774d586ab6d6ad3289e22c87f
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>