ASoC: rt5640: Fix ignored error checks

The negative error value returned from get_sdp_info() is ignored
because it's assigned to unsigned variables.

Spotted by coverity CIDs 1042657, 1042658.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@linaro.org>