MdePkg: Correct Help of Error Level

MdePkg: Correct Help of Error Level

1. Error Level should be BIT31 instead of BIT28.
2. New PCD PcdFixedDebugPrintErrorLevel value should be mask value of all BITs so that it doesn't bring impact for current platform.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16799 6f19259b-4bc3-4df7-8a09-765794883524