Having the check in a centralized place makes the code a bit cleaner and
shorter.

Note: There is a slight semantic change in this patch. soc_probe_aux_dev() will
no longer return -EBUSY if the AUX dev has already been probed before. This is
fine though since it will simply do nothing in that case and return success.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>