msm_serial_hs: Wait for discard flush completion for UART Rx channel

Completion of requested flush command with ADM driver is having more
latency then previously. Hence now it is required to wait for discard
flush complete if there is more events expected with UART driver which
would go out of sync without it. Below are 2 instances where it is must
to wait for discard flush completion requested on UART Rx channel.

1. Changing Baud Rate of UART
UART application can send baud rate change request based on its requirement
of communication with connected device on remote uart. Serial core also
does set by default baud rate when application is opening the uart port. As
Rx command is queued always with ADM driver from UART driver, for above
events it is required to flush the same after setting the baud rate. Not
waiting for completion of Rx flush would allow application to send command
on Tx or any other ioctl which would reach to connected device but response
may not be received as Rx flush completion is not received to queue next Rx
command to ADM. Hence with this there are chances that received data with
UART wil be lost when Rx flush completion is not received in-time. Hence
wait for Rx flush completion from set_termios() with timeout as 300 jiffie.
Rx flush completion time is non-deterministic as it depends on number of
commands queued to ADM driver from ADM client drivers.

2. While going for UART clock off
UART clock off has multiple state machines and before going into last
state it makes sure that there are no pending data in UART Tx and Rx FIFO.
Although there would be one Rx command queued which is flushed before
moving to last state and going ahead with UART clock off. If UART clock is
turned off before Rx flush request is being executed and ADM tried to flush
Rx command, ADM encounters data bus error on UART Rx Channel. Hence with
waiting for Rx flush completion, and then doing UART clock off on receving
the same would resolve ADM data bus error issue on UART Rx channel.

CRs-Fixed: 457769
Change-Id: I9344277224a2dc3f28140a1f42aa3feed5411b97
Signed-off-by: Mayank Rana <mrana@codeaurora.org>