gpio: sim: use sysfs_streq() and avoid an strdup()

When comparing strings passed to us from configfs, we can pass the page
argument directly to sysfs_streq() and avoid manual string trimming.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>