MdeModulePkg/PciSioSerial: Fix a bug that wrongly produces 2 UARTs

MdeModulePkg/PciSioSerial: Fix a bug that wrongly produces 2 UARTs

When PciSioSerial is firstly started with a non-NULL remaining
device path, the UART instance is created using the parameters
specified in the remaining device path. Later when the driver
is started again on the same UART controller with NULL remaining
device path, the correct logic is to directly return SUCCESS
instead of current buggy implementation which wrongly produces
another UART using the default parameters.

The bug causes two UARTs are created when the UART is configured
in 57600 baud rate.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>