opp: Replace ENOTSUPP with EOPNOTSUPP

Checkpatch gives following warning for new patches, and the new patches
normally follow the existing standards for such stuff. Lets fix it
properly.

WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>