regulator: fan53880: Add support for COMPILE_TEST

This patch adds support for COMPILE_TEST while fixing a warning when
no support for device tree is there.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Link: https://lore.kernel.org/r/1c437154873ace65ff738a0ebca511308f1cecc1.camel@googlemail.com


Signed-off-by: Mark Brown <broonie@kernel.org>