nvmem: imx-ocotp: Fix return value of imx_ocotp_read

imx_ocotp_read() should return 0 on success.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>