rmnet_core: check tcp flags when segmenting packets

Now that the hardware will coalesce additional flags into the RSC frame,
we must check the TCP flags explicitly and remove ones that make no sense
when segmenting the RSC frame for any reason.

In particular, we must check for TCP_FIN, and clear this flag from any
segmetents of the RSSC frame except the last to avoid breaking the TCP
stream integrity.

Change-Id: I7f80437991821ab29cd190a9d5bd45e9cdb4c333
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>