ethtool: Implement ethtool_puts()

Use strscpy() to implement ethtool_puts().

Functionally the same as ethtool_sprintf() when it's used with two
arguments or with just "%s" format specifier.

Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Madhuri Sripada <madhuri.sripada@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>