regulator: tps51632: Fix setting ramp delay

According to the datasheet:

SLEW Register(Address = 07h)
b7      b6      b5      b4      b3      b2      b1      b0
48mV/us 42mV/us 36mV/us 30mV/us 24mV/us 18mV/us 12mV/us 6mV/us

Current code does not set correct slew rate in some cases:
e.g. Assume ramp_delay is 10000, current code sets slew register to 6mV/us.
Fix the logic to set slew register.

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