Correct the convert hex string to decimal value logic.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/branches/UDK2014.SP1@16151 6f19259b-4bc3-4df7-8a09-765794883524