sfc/ethtool_common: Make some function to static

Fix sparse warning:

drivers/net/ethernet/sfc/ethtool_common.c
  warning: symbol 'efx_fill_test' was not declared. Should it be static?
  warning: symbol 'efx_fill_loopback_test' was not declared.
           Should it be static?
  warning: symbol 'efx_describe_per_queue_stats' was not declared.
           Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>