sfc: Use strlcpy() to copy ethtool stats names

Fix CID 113703 in the Coverity report on Linux.

ethtool stats names are limited to 32 bytes including a null
terminator.  Use strlcpy() to ensure that we will always include the
null terminator even if a source string becomes longer than this.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>