slim-msm: manage TX message queue pointer

In the TX message queue handler, update the TX head such that
it correctly points to the next message to be processed. Fix
the logic such that all the processed IO vectors are handled, even
when the packets arrive out of order.

Change-Id: Iee45c5acb45f9a8eb475667cc4a87d13b62adc7a
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>