SecurityPkg/SecureBootConfigDxe: Change the declaring of buffer.

SecurityPkg/SecureBootConfigDxe: Change the declaring of buffer.

The change doesn't impact the functionality.
To avoid magic code is helpful for maintaining the codes.
Use stack variable for known max length variable is more
clear and safe than heap buffer.

Cc: Zhang Chao B <chao.b.zhang@intel.com>
Cc: Wu Hao A <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: chenc2 <chen.a.chen@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>