msm_serial_hs: Improve UART Shutdown path

For uart port close operation, serial core calls msm_hs_stop_rx_locked()
which sends discard flush request to ADM driver after changing rx.flush
state machine from FLUSH_NONE to FLUSH_STOP value. Next serial core
calls msm_hs_shutdown()and it waits for discard flush completion with
rx.flush state machine to be set as FLUSH_SHUTDOWN to complete uart
port close operation. With this approach, transfer termination is not
known by UART Core which may still allow UART core to make CRCI request
with ADM Core if new data is being received from remote uart. Hence
invoke Force RxStale from msm_hs_stop_rx_locked() which informs UART
core to terminate current Rx transfer and do not make any new CRCI
request with ADM core for any new incoming data. On receiving RxStale
interrupt, it sends discard flush request to ADM driver to terminate
transfer with ADM hardware.

This new programming sequence is required to get both UART and ADM
hardwares in-sync for termination of transfer.

CRs-Fixed: 475230
Change-Id: I802a456294d4fd2fa51a5f4e918a4f4f22bc5ee3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>