ASoC: wm2000: Remove wm2000_read helper function

The return type "unsigned int" was used by the wm2000_read()
function despite of the aspect that it will eventually return
a negative error code.
The resulting function doesn't add much to the code, so replace
wm2000_read with regmap_read.

Signed-off-by: Lucas Tanure <tanureal@opensource.cirrus.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>