PM / OPP: Fix potential access to invalid list entry

If struct device_opp has one entry in its opp_list, and a new_opp entry
with a larger rate is added, an unintended memory access occurs.
Fix this issue by only accessing opp->rate when opp->node is a valid list
item.
Discovered with KASAN.

Change-Id: Ib3b75324e349897c25ed7e54ef107700ebdc60c2
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>