MdeModulePkg PeiCore: Fix VS2012 build failure

MdeModulePkg PeiCore: Fix VS2012 build failure

fwvol.c(1572) : warning C4701: potentially uninitialized
local variable 'Status' used

The build failure is caused by
0e042d0ad76157ac9bad17bb4e1ff2919ca0d8f4 for
https://bugzilla.tianocore.org/show_bug.cgi?id=1131

This patch initializes Status to fix the build failure.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>