serail: msm_geni_serial: Stop TX Engine during port close

This change is added to stop_tx during port shutdown.

Leaving TX sequencer active after port shutdown resulting
in unexpected behaviour because we expect TX sequencer
and DMA engine to go IDLE after port close.

In some cases TX remains active and trying to send invalid
TX zero bytes over TX line. Hence stop TX during port close.

Change-Id: I5ac15726bee16d2cc4de96dcd64e5c1d2b743908
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>