net: qualcomm: rmnet: Don't reference skb->data directly

In preparation for receiving non-linear SKBs from lower level drivers,
don't access skb->data in functions that will be used on the RX path
unless we know the SKB is linear.

Change-Id: I99429550966d4384d0725b9c72662469a1036552
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>