lan78xx: 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.

Other implementations of .*get_drvinfo use strscpy so this patch brings
lan78xx_get_drvinfo() in line as well:

igb/igb_ethtool.c +851
static void igb_get_drvinfo(struct net_device *netdev,

igbvf/ethtool.c
167:static void igbvf_get_drvinfo(struct net_device *netdev,

i40e/i40e_ethtool.c
1999:static void i40e_get_drvinfo(struct net_device *netdev,

e1000/e1000_ethtool.c
529:static void e1000_get_drvinfo(struct net_device *netdev,

ixgbevf/ethtool.c
211:static void ixgbevf_get_drvinfo(struct net_device *netdev,

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231012-strncpy-drivers-net-usb-lan78xx-c-v1-1-99d513061dfc@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>