ShellPkg/UefiDpLib: Fix the error message "Timer library instance error!"

When executing shell dp command, there is an error message "Timer library
instance error!"

The error message "Timer library instance error!" should be for the case about
duration > EndTimeStamp if CountUp or duration > StartTimeStamp if CountDown.

But if the EndTimeStamp of an entry is not added, it should not the case to catch.

This change fixes the error message "Timer library instance error!" from the
"BdsAttempt" entry which is logged when trying to boot a boot option.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Cinnamon Shia <cinnamon.shia@hpe.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>