PM / devfreq: Replace strncpy with strscpy

GCC produces this warning when kernel compiled using `make W=1`:

  warning: ‘strncpy’ specified bound 16 equals destination size [-Wstringop-truncation]
  772 |  strncpy(devfreq->governor_name, governor_name, DEVFREQ_NAME_LEN);

The strncpy doesn't take care of NULL-termination of the destination
buffer, while the strscpy does.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>