regulator: twl-regulator: Fix off-by-one vsel setting in twl6030smps_set_voltage

commit 268a1 "regulator: twl-regulator: Use DIV_ROUND_UP at appropriate places"
introduced an off-by-one bug for setting vsel.

The linear calculation code in twl6030smps_list_voltage() does subtract index
by 1 so we need the vsel++ after DIV_ROUND_UP.

The original code use this trick to differentiate if we are going to set the
voltage to 0 or the voltage falls within the linear calculation range.

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