efi/libstub: Use C99-style for loop to traverse handle buffer

Tweak the for_each_efi_handle() macro in order to avoid the need on the
part of the caller to provide a loop counter variable.

Also move efi_get_handle_num() to the callers, so that each occurrence
can be replaced with the actual number returned by the simplified
LocateHandleBuffer API.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>