IpmiFeaturePkg: Refine code to avoid warning report

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=4522



"if (FixedPcdGet8 (PcdKcsInterfaceSupport) == 1) {" will not be executed in current code,
put it within "case SysInterfaceKcs:" to avoid this issue.

Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>

Signed-off-by: Yanbo Huang <yanbo.huang@intel.com>