regulator: da9063: fix null pointer deref with partial DT config

regulator: da9063: fix null pointer deref with partial DT config

When some of the da9063 regulators do not have corresponding DT nodes
a null pointer dereference occurs on boot because such regulators have
no init_data causing the pointers calculated in
da9063_check_xvp_constraints() to be invalid.

Do not dereference them in this case.

Fixes: b8717a80e6ee ("regulator: da9063: implement setter for voltage monitoring")
Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Link: https://lore.kernel.org/r/20230616143736.2946173-1-martin.fuzzey@flowbird.group
Signed-off-by: Mark Brown <broonie@kernel.org>