regulator: qcom_spmi: Do NULL check for lvs

Low-voltage switches (lvs) don't have set_points since the voltage ranges
of the output are really controlled by the inputs.  This is a problem for
the newly added linear range support in the probe(), as that will cause
a null pointer dereference error on older platforms like msm8974 which
happen to need to control some of the implemented lvs.

Fix this by adding the appropriate null check.

Fixes: 86f4ff7a

 ("regulator: qcom_spmi: enable linear range info")
Reported-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
Tested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>