IpmiFeaturePkg: refine GetSelfTest function

Use predefined type while accessing IPMI command returned data
instead of raw byte array.

Signed-off-by: Mike Maslenkin <mike.maslenkin@gmail.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>