regulator: MAX8998: set_voltage bugfix. ramp_up delay and min/max voltage

Two issues are addressed for max8998_set_voltage function.

1. Min/Max Voltage.

	max8998_set_voltage had been using the voltage value of

	min ( voltage[i] >= max_vol , i )

	This is corrected to use:

	min ( voltage[i] >= min_vol , i )

2. Ramp Up Delay.

	max8998_set_voltage should provide delay for BUCK1/2
if ENRAMP is on. It reads RAMP value from ONOFF4 register to determine
RAMP delay length. However, when max8998_set_voltage's new voltage is
lower than the previous, we don't care because it does not deteriorate
the stability.

Changes since v1:
- rebased onto latest regulator-for-next tree

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>