regulator: rtq2208: Fix the BUCK ramp_delay range to maximum of 16mVstep/us

The maximum ramp up and down range of BUCK are shorten
from 64mVstep/us to 16mVstep/us.
Therefore, the RTQ2208_RAMP_VALUE_MAX_uV is modified
to 16000uV in this version.

Signed-off-by: Alina Yu <alina_yu@richtek.com>
Link: https://lore.kernel.org/r/1777b18c0faa1f6b7196630e679a03362686c301.1714467553.git.alina_yu@richtek.com
Signed-off-by: Mark Brown <broonie@kernel.org>