regulator: Fix get_mode() for WM835x DCDCs

The WM835x regulators need a different register checking for force
mode on each DCDC. Previously the force mode status for DCDC1 was
checked.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>