opp: Drop unnecessary check from dev_pm_opp_attach_genpd()

Since commit c0ab9e08 ("opp: Allocate genpd_virt_devs from
dev_pm_opp_attach_genpd()"), the allocation of the virtual devices is
moved to dev_pm_opp_attach_genpd() and this check isn't required anymore
as it will always fail. Drop it.

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