EmulatorPkg/MpService: fix wrong unsigned to signed variable transition
    
    Because TimeoutInMicrosecsond is a unsigned value, converting it to
    signed value will cause the data region changed. so this patch fix
    that.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    
    git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16415 6f19259b-4bc3-4df7-8a09-765794883524