greybus: arche-apb: NULL is a valid regulator

Since NULL could in theory be a valid regulator we ought to check for
IS_ERR() rather than for NULL. In practice this is unlikely to be an
issue but it's better for neatness.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>