regulator: Ensure val is initialised in 88pm8607 choose_voltage()

If we fall through it means that we hit an unknown regulator/chip
combination so set -ENOENT as an explicit flag (the return code
is only used internally).

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>