efi_loader: initialize return values in efi_uninstall_multiple_protocol_interfaces_int()

If the va_list we got handed over contains no protocols we must return
EFI_SUCCESS.  However in that case the current code just returns
an unintialized value.
Fix that by setting the return value in the variable definition

Addresses-Coverity: CID 376195:  ("Uninitialized variables  (UNINIT)")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>