PM / devfreq: fix sscanf handling for writable sysfs entries

sscanf returns 0 when an invalid parameter like:
echo -n "a">min_freq
is attempted. Returning back the return result(0) will cause the command not to return back to command prompt.

Instead, just return -EINVAL when sscanf does not return 1.

This is done for min_freq, max_freq and polling_interval

Cc MyungJoo Ham <[email protected]>

Cc Kyungmin Park <[email protected]>

Cc "Rafael J. Wysocki" <[email protected]>

Cc Kevin Hilman <[email protected]>

Cc [email protected]

Cc [email protected]

Signed-off-by Nishanth Menon <[email protected]>