devfreq: check if get_dev_status is implemented before calling it

devfreq governor code calls the devfreq profile's callback
get_dev_status to return the device status. Not all devices
implement this optional callback. Hence check if the callback
is present before accessing it.

Change-Id: I8fe8ff367ccdcb82ad686e47186258c4cd086696
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>
Signed-off-by: Rama Aparna Mallavarapu <aparnam@codeaurora.org>