opp: Change return type of devm_pm_opp_attach_genpd()

Make devm_pm_opp_attach_genpd() to return error code instead of
opp_table pointer in order to have return type consistent with the
other resource-managed OPP helpers.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>