msm: rmnet_bam: fix xmit statistics

The skb being transmitted can be freed at any point after handing it
off to the transport.  Thus the statistics must be determined in the
callback to prevent accessing data that is already freed.  Also,
statistics should always be calculated when in IP mode.

CRs-Fixed: 318747
Change-Id: I3a2a081291fc657ca64aff47a3370017c5b6f7ca
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>