regulator: palmas: Fix off-by-one for ramp_delay and register value mapping

Datasheet says:
Time Step (TSTEP) selection, when changing the output voltage, the new value is
reached through successive voltage steps (if not bypassed). The equivalent
programmable slew rate of the output voltage is:

TSTEP[1:0]: 00 Jump (no slope control)
TSTEP[1:0]: 01 10mV/us
TSTEP[1:0]: 10 5mV/us (default)
TSTEP[1:0]: 11 2.5mV/us

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>