regulator: lp3971: 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 BUCK_TARGET_VOL_MAX_IDX in
lp3971_dcdc_get_voltage_sel.

BUCK_TARGET_VOL_MIN_IDX and BUCK_TARGET_VOL_MAX_IDX are not used, remove them.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>