regulator: max8973: Don't override control1 variable when set ramp delay bits

Current code overrides control1 variable when setting ramp delay bits.
Fix it by just setting ramp_delay bits.

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>