regulator: max8973: Fix setting ramp delay

Current code can set ramp delay to a wrong setting that the return value
from .set_voltage_time_sel is not enough for proper delay.
Fix the logic in .set_ramp_delay and also remove unused ret_val variable.

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