If 'ReadKeyStroke' function return EFI_NOT_READY then skip it.
If the return value is EFI_DEVICE_ERROR clean the currentString buffer.
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Pedroa Liu <pedroa.liu@insyde.com>
Reviewed-by: Qiu Shumin <shumin.qiu@intel.com>