devfreq: Do not allow tunable updates when device is suspended

Tunables update accesses the monitor's registers for some devices.
When the device is suspended, the clocks for those devices are turned
off and updating the tunables of a suspended device causes the device
to crash. Fix this by checking the flag dev_suspended before allowing
the tunable updates.

Change-Id: I3a0938edf761a9ea6d39ffeec19f35c228d5c306
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>