regulator: palmas: Don't update tstep register for SMPS3 and SMPS7

SMPS3 and SMPS7 do not have tstep_addr setting, so current code actually
writes 0 to smps12_ctl (offset is 0) register when set_ramp_delay callback
is called for SMPS3 and SMPS7.

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>