regulator: Fix check of unsigned return value and transmit errors in wm831x_gp_ldo_get_mode()

If ret is unsigned, the checks for negative wm831x_reg_read() return values
are wrong. The error value should be transmitted to its caller, e.g.
wm831x_gp_ldo_get_status() which tests for a negative return value.

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>