MdeModulePkg: PEI Core: clean up "AprioriFile" handling in FindFileEx()

MdeModulePkg: PEI Core: clean up "AprioriFile" handling in FindFileEx()

Clean up two issues around FindFileEx():

- The "AprioriFile" parameter's type differs between the function
  declaration and the function definition. The correct type is
  (EFI_PEI_FILE_HANDLE*).

- "FfsFileHeader" has type (EFI_FFS_FILE_HEADER*); for clarity, we should
  cast it explicitly to EFI_PEI_FILE_HANDLE when assigning it to
  (*AprioriFile).

This is a semantic cleanup, there is no functional change.

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>