Once the data is sent, we need to preserve the full frame for
the ndlc state machine. If the NDLC ACK is not received in time,
the ndlc layer will resend the same frame.
Having the header byte pulled will corrupt the frame.

Cc: stable@vger.kernel.org
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>