OPP: Return correct value in dev_pm_opp_get_level

OPP: Return correct value in dev_pm_opp_get_level

Commit 073d3d2ca7d4 ("OPP: Level zero is valid") modified the
documentation for this function to indicate that errors should return a
non-zero value to avoid colliding with the OPP level zero, however
forgot to actually update the return.

No in-tree kernel code depends on the error value being 0.

Fixes: 073d3d2ca7d4 ("OPP: Level zero is valid")
Signed-off-by: Aleks Todorov <aleksbgbg@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>