gpiolib: cdev: replace strncpy() with strscpy()

Replace usage of strncpy() with strscpy() to remove -Wstringop-truncation
warnings.

The structures being populated are zeroed, to prevent stack leakage as
they are returned to userspace, so strscpy() performs the equivalent
function without the warnings.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>