regulator: tps6586x: Remove unnecessary rdev[] array

Now we are using devm_regulator_register(), so we don't need the rdev[] array
to store return value of devm_regulator_register. Use a *rdev variable is
enough for checking return status.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>