MdeModulePkg/SerialDxe: Fix not able to change serial attributes

MdeModulePkg/SerialDxe: Fix not able to change serial attributes

Issue : When try to change serial attributes using sermode
command, the default values are set with the execute flow
as below.

The sermode command calls SerialSetAttributes, which sets H/W
attributes of Serial device. After that the SerialIo protocol is
reinstalled, which causes MdeModulePkg/Universal/Console/TerminalDxe
and MdeModulePkg/Universal/Console/ConPlatformDxe drivers' bindings
to stop and then start. This in turn calls SerialReset, which undoes
changes of SerialSetAttributes.

Cause : The SerialReset command resets the attributes' values
to default.
Fix : Serial Reset command should set the attributes which have
been changed by user after calling SerialSetAttributes.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>