net: mdio-gpio: replace deprecated strncpy with strscpy

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.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231211-strncpy-drivers-net-mdio-mdio-gpio-c-v3-1-76dea53a1a52@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>