PM / OPP: silence an uninitialized variable warning

Smatch complains that it's possible we print "rate" in the debug output
when it hasn't been initialized.  It should be zero on that path.

Fixes: a1e8c136 ("PM / OPP: "opp-hz" is optional for power domains")
[ Viresh: Added the Fixes tag ]
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>