net: macb: use ethtool_sprintf to fill ethtool stats strings

net: macb: use ethtool_sprintf to fill ethtool stats strings

The RISC-V toolchain triggers a stringop-truncation warning when using
snprintf() with a fixed ETH_GSTRING_LEN (32 bytes) buffer.

Convert the driver to use the modern ethtool_sprintf() API from
linux/ethtool.h. This removes the need for manual snprintf() and
memcpy() calls, handles the 32-byte padding automatically, and
simplifies the logic by removing manual pointer arithmetic.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Sean Chang <seanwascoding@gmail.com>
Link: https://patch.msgid.link/20260302142931.49108-1-seanwascoding@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>