msm: clock-rpm: Return cached rate when the RPM doesn't support querying

On the MSM Copper, the RPM processor does not support
the ability to query the state of a resource. Return
the cached rate in the clock struct instead of attempting
to query the RPM. This will also allow debugfs to become
useful again for RPM clocks.

Change-Id: If3e0cde516a96130ac63b5e6b832852940fe5764
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>