IpmiFeaturePkg: fix IPMI GetSelfTest command response parsing

Byte 0 of a response contains completion code for the command.
So, the examined data starts from byte 1. It's easy to make a mistake
here since specification counts response data from 1.

For the "Get Self Test Results" command Intelligent Platform Management
Interface Specification v2.0 rev 1.1 paragraph 20.4 defines response as:
+-----+---------------------------------------------------------------+
|byte | data field                                                    |
+-----+---------------------------------------------------------------+
| 1   | Completion Code                                               |
|     |                                                               |
| 2   | 55h =  No error. All Self Tests Passed.                       |
|     | 56h = Self Test function not implemented in this controller.  |
|     | 57h = Corrupted or inaccessible data or devices               |
|     | 58h = Fatal hardware error                                    |
|     |                                                               |
| 3   | For byte 2 = 55h, 56h, FFh: 00h                               |
|     | For byte 2 = 58h, all other: Device-specific                  |
|     | For byte 2 = 57h: self-test error bitfield.                   |
+-----+---------------------------------------------------------------+

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>