serial: msm_geni_serial: Ensure Flow register is written during suspend

Before shutting down the Rx engine as part of suspending the device,
ensure that the manual flow control register is written to and gets time
to take effect before proceeding with the rest of the Rx shutdown
sequence. Shutting down the Rx engine prematurely before the flow register
has had a chance to take effect could flow off the peer.

Change-Id: I7fc0d25840602e4795f720e26845092ff5ea2804
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>