MdeModulePkg/Ps2Kb: ReadKeyStrokeEx always return key state

MdeModulePkg/Ps2Kb: ReadKeyStrokeEx always return key state

Today's implementation only return key state when there is key.
But when user doesn't press any key, the key state cannot be
returned.

The patch changes the ReadKeyStrokeEx() to always return the
key state even there is no key pressed.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>