mfd: wm8400-core: Delete wm8400_reg_read()

There was a static checker warning in wm8400_reg_read() because we were
returning u16 and that can't hold the negative error codes.  The
function isn't used, so let's just delete it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>