efi_loader: error handling in efi_disk_add_dev

* If an error occurs in efi_disk_add_dev(), don't leak resources.
* If calloc() fails while creating the file system protocol interface,
  signal an error.
* Rename efi_simple_file_system() to efi_create_simple_file_system().
* Drop a little helpful debug message.

Fixes: 2a92080d ("efi_loader: add file/filesys support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>