msm_serial_hs: Add flush state machine for UART Tx Transfer

Handle below error cases with UART Tx transfer:
1. Application doing I/O to UART port and is being terminated abnormally.
2. Used ADM Channel for UART TX transfer stalls.

In both above conditions, queued Tx command to ADM is not flushed. Hence
restarting of application shows list corruption for queueing next UART
Tx command to ADM. This change fixes it by having state machine for UART
Tx, identifying such condition and flushing the queue Tx command before
closing the UART device.

Change-Id: I4e567a539d890dffed50b5837dde58d5cd62d641
Signed-off-by: Mayank Rana <mrana@codeaurora.org>