devfreq: check for valid hwmon node during get frequency

Update the condition to check valid hwmon before accessing it because
during parallel usecase of resume call and scaling_max_freq store call
it triggers a call to governor to get frequency and governor tries to
access hwmon node from devfreq pointer and it will be NULL if the
governor has failed to start.

Change-Id: I6ee7b784c3cf5e62d5f3f5b48e75ed44483d1a50
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>