regulator: lp872x: Simplify implementation of lp872x_find_regulator_init_data()

Pass regulator id rahter than the index to lp872x_find_regulator_init_data(),
then the code can be simpler.
We can also get max_regulators by lp->num_regulators.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Tested-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>