opp: Fix some kernel-doc comments

Make @freq to @bw in dev_pm_opp_find_bw_ceil() and
dev_pm_opp_find_bw_floor() kernel-doc comment to
remove warnings found by running scripts/kernel-doc,
which is caused by using 'make W=1'.

drivers/opp/core.c:753: warning: Function parameter or member 'bw' not
described in 'dev_pm_opp_find_bw_ceil'
drivers/opp/core.c:753: warning: Excess function parameter 'freq'
description in 'dev_pm_opp_find_bw_ceil'
drivers/opp/core.c:812: warning: Function parameter or member 'bw' not
described in 'dev_pm_opp_find_bw_floor'
drivers/opp/core.c:812: warning: Excess function parameter 'freq'
description in 'dev_pm_opp_find_bw_floor'

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>