regulator: wm831x_reg_read() failure unnoticed in wm831x_aldo_get_mode()

ret should be signed to notice a failure in wm831x_reg_read().

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>