opp: of: drop incorrect lockdep_assert_held()

_find_opp_of_np() doesn't traverse the list of OPP tables but instead
just the entries within an OPP table and so only requires to lock the
OPP table itself.

The lockdep_assert_held() was added there by mistake and isn't really
required.

Fixes: 5d6d106f ("OPP: Populate required opp tables from "required-opps" property")
Cc: v5.0+ <stable@vger.kernel.org> # v5.0+
Reported-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>