disk: Tidy up #ifdefs in part_efi

This file does not correctly handle the various cases, sometimes
producing warnings about partition_basic_data_guid being defined but not
used. Fix it.

There was some discussion about adjusting Kconfig or making
HAVE_BLOCK_DEVICE a prerequisite for PARTITIONS, but apparently this is
not feasible. Such changes can be undertaken separate from the goal of
this series.

Signed-off-by: Simon Glass <sjg@chromium.org>