MdeModulePkg/Udf: Avoid declaring and initializing local GUID variable

MdeModulePkg/Udf: Avoid declaring and initializing local GUID variable

The local GUID variable 'UdfDevPathGuid', it has been initialized during
its declaration.

For better coding style, this commit uses a global variable instead.

Cc: Paulo Alcantara <pcacjr@zytor.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <pcacjr@zytor.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>