strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.
We expect new_bus->id to be NUL-terminated but not NUL-padded based on
its prior assignment through snprintf:
| snprintf(new_bus->id, MII_BUS_ID_SIZE, "gpio-%x", bus_id);
Due to this, a suitable replacement is `strscpy` [2] due to the fact
that it guarantees NUL-termination on the destination buffer without
unnecessarily NUL-padding.
We can also use sizeof() instead of a length macro as this more closely
ties the maximum buffer size to the destination buffer. Do this for two
instances.