OPP: OF: Fix an OF node leak in _opp_add_static_v2()

_opp_add_static_v2() leaks the obtained OF node reference when
_of_opp_alloc_required_opps() fails. Add an of_node_put() call in the
error path.

Fixes: 3466ea2c ("OPP: Don't drop opp->np reference while it is still in use")
Signed-off-by: Joe Hattori <joe@pf.is.s.u-tokyo.ac.jp>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>