regulator: mc13xxx: Fix NULL pointer error in non-DT mode

Add a check to avoid NULL pointer dereference error when
booted in non-DT mode. While at it also remove the additional
of_node_get which is no longer needed for of_get_child_by_name
and fix the node pointer.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reported-by: Philippe Rétornaz <philippe.retornaz@epfl.ch>
Signed-off-by: Mark Brown <broonie@linaro.org>