regulator: tps6507x: Use "IS_ENABLED" for DT code.

Instead of "#ifdef CONFIG_OF" use "IS_ENABLED(CONFIG_OF)"
option for DT code to avoid if-deffery in code.
Also, modify code as per coding style.

Signed-off-by: Manish Badarkhe <badarkhe.manish@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>