regulator: tps65910: Add missing breaks in switch/case

Also add a default case in tps65910_list_voltage_dcdc to silence
'volt' may be used uninitialized in this function warning.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>