MdeModulePkg: ConfigKeywordHandler: fix pointer target signedness.

MdeModulePkg: ConfigKeywordHandler: fix pointer target signedness.

SVN r17428 ("MdeModulePkg: Implement UEFI25 HII Config keyword handler protocol") introduced code that triggers

  -Werror=pointer-sign

under gcc ("warn for pointer argument passing or assignment with different signedness"). This patch fix up those locations.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17445 6f19259b-4bc3-4df7-8a09-765794883524