net: qualcomm: rmnet: Fix skb->csum_start value

Now that we can reuse the original SKB from the lower level drivers
during RSB/RSC processing, we can no longer assume that the transport
header offset is set when stamping the GSO information.

Change-Id: I017c089bfc99eae6a07b2f60632c9e4ffac89de1
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>