msm_serial_hs: fix tx state machine

Update the in-flight tx indicator upon receiving
the start_tx call from the framework such that
subsequent requests get queued properly.

Change-Id: Ia6f0c0277ca9136e2a79101d93b0c9b802f0639d
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>