efi_stub: Use efi_uintn_t

Commit f5a2a938 ("efi_loader: consistently use efi_uintn_t in boot
services") changed the internal EFI API header without adapting its existing
EFI stub users. Let's adapt the EFI stub as well.

Fixes: f5a2a938 ("efi_loader: consistently use efi_uintn_t in boot services")
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>