MdeModulePkg/PciSioSerialDxe: Flush Tx before config change

MdeModulePkg/PciSioSerialDxe: Flush Tx before config change

https://bugzilla.tianocore.org/show_bug.cgi?id=3114

Add logic to flush all UART transmit buffers if there is a
config change from Reset(), SetAttributes() or SetControl().
Use a timeout in the flush operation, so the system can
continue to boot if the transmit buffers can not be
flushed for any reason.

This change prevents lost characters on serial debug logs
and serial consoles when a config change is made.  It also
prevents a UART from getting into a bad state or reporting
error status due to characters being transmitted at the same
time registers are updated with new communications settings.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>