These functions call platform's sysctl getter and setters.

Note that the called platform functions are called nm_platform_sysctl_get()
and nm_platform_sysctl_set(). Also, in this case they use the ip-conf path
via nm_utils_sysctl_ip_conf_path().

Also, next we will add API nm_platform_sysctl_ip_conf_get() and
nm_platform_sysctl_ip_conf_set(), which will be wrappers around
nm_platform_sysctl_get() and nm_platform_sysctl_set(), using the ip-conf
paths as well.

Rename the device functions, to be more similar to the existing and future
naming in platform.