regulator: 88pm8607: Fix writting value to vol_reg in pm8607_set_voltage_sel

commit 4ca1e1d "regulator: Convert 88pm8607 to set_voltage_sel" accidentally
changed the value writing to vol_reg.
What we want is to write val instead of selector to vol_reg.

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