regulator: Convert tps65023 to set_voltage_sel

This patch also changes the define for TPS65023_LDO_CTRL_LDOx_MASK,
because regmap_update_bits actually does:
        (orig & ~mask) | (val & mask)
This change makes the code consists for all regmap_update_bits calls.

Also removes some uninformative comments.

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