regulator: lp3972: Convert to get_voltage_sel

regulator_list_voltage_table() returns -EINVAL if selector >= n_voltages.
Thus we don't need to check if reg is greater than LP3972_BUCK_VOL_MAX_IDX in
lp3972_[ldo|dcdc]_get_voltage_sel.

LP3972_BUCK_VOL_MIN_IDX and LP3972_BUCK_VOL_MAX_IDX are not used, remove them.

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