regulator: da903x: Don't read/write to DA9030_INVAL/DA9034_INVAL address

For fixed voltage, DA9030_LDO13 and DA9034_LDO5, the info->vol_reg is
DA9030_INVAL/DA9034_INVAL.
It does not make sense to read/write to DA9030_INVAL/DA9034_INVAL address.

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