regulator: lp8788-ldo: Set n_voltages to 1 for fixed voltage

For fixed voltage, the n_voltages should be 1 rather than 0.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>