efi_loader: only check size if EFI_DT_APPLY_FIXUPS

In the implementation of the EFI_DT_FIXUP_PROTOCOL:

* Only check the buffer size when EFI_DT_APPLY_FIXUPS is set.
* In this case the field totalsize of the device-tree may not exceed the
  buffer size.
* Install device-tree only if EFI_DT_INSTALL_TABLE is set.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>