opp: Fix error check in dev_pm_opp_attach_genpd()

dev_pm_domain_attach_by_name() may return NULL in some cases,
so IS_ERR() doesn't meet the requirements. Thus fix it.

Fixes: 6319aee1

 ("opp: Attach genpds to devices from within OPP core")
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
[ Viresh: Replace ENODATA with ENODEV ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>