OPP: Fix an un-initialized variable usage

smatch complains that 'ret' may be returned un-initialized.

Explicitly return 0 if we reach the end of the function (should
'opp_table->clk_count' be 0).

Fixes: 8174a3a6 ("OPP: Provide a simple implementation to configure multiple clocks")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>