IntelSiliconPkg/PlatformVTdSample: Avoid using constant result 'if'

IntelSiliconPkg/PlatformVTdSample: Avoid using constant result 'if'

In this sample driver, if (0) {...} else {...} statements were used to
illustrate two different using scenarios.

This comment refines the coding style by substituting the 'if (0)'
statement with comments to select sample codes for different cases.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>