ASoC: wm_hubs: Silence reg_r and reg_l 'may be used uninitialized' warnings

Return an error from wm_hubs_read_dc_servo() if hubs->dcs_readback_mode is not
correctly initialized. You might as well bail out since nothing is likely to
work correctly afterwards.

sound/soc/codecs/wm_hubs.c:321:11: warning: 'reg_r' may be used uninitialized in this function [-Wuninitialized]
sound/soc/codecs/wm_hubs.c:251:13: note: 'reg_r' was declared here
sound/soc/codecs/wm_hubs.c:322:11: warning: 'reg_l' may be used uninitialized in this function [-Wuninitialized]
sound/soc/codecs/wm_hubs.c:251:6: note: 'reg_l' was declared here

gcc version 4.6.3

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>