MdePkg/Ftp4: Fix wrong function pointer declaration

EFI_FTP4_DATA_CALLBACK is a function pointer and defined as follows:

> typedef
> EFI_STATUS
> (EFIAPI *EFI_FTP4_DATA_CALLBACK)(
>  IN EFI_FTP4_PROTOCOL           *This,
>  IN EFI_FTP4_COMMAND_TOKEN      *Token
>  );

And EFI_FTP4_COMMAND_TOKEN structure declared it as:

> EFI_FTP4_DATA_CALLBACK *DataCallback

Which ended up being a pointer to function pointer and clearly wrong.
This patch fixes it by removing the misleading '*' from declaration.
It's also fixed in new UEFI 2.7 spec.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Paulo Alcantara <paulo.alc.cavalcanti@hp.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>