iio: ms65611_core: Fixes dereferencing regulator pointer

Change in handling of the regulator description means that static
checkers correctly assume we could be using dereferenced pointer to the
regulator. In reality we will never get the -ENODEV error, as current
behavior flow does not predict it, because:
If the device tree or board file does not define suitable regulators for
the component, it will be substituted by a dummy regulator, or, if
regulators are disabled altogether, by stubs.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Crt Mori <cmo@melexis.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>