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 <[e-mail]>
Signed-off-by: Zhang Xiaoxu <[e-mail]>
Reviewed-by: Martin Habets <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/sfc/ethtool_common.c

···
  *
  * Fill in an individual self-test entry.
  */
-void efx_fill_test(unsigned int test_index, u8 *strings, u64 *data,
-		   int *test, const char *unit_format, int unit_id,
-		   const char *test_format, const char *test_id)
+static void efx_fill_test(unsigned int test_index, u8 *strings, u64 *data,
+			  int *test, const char *unit_format, int unit_id,
+			  const char *test_format, const char *test_id)
 {
 	char unit_str[ETH_GSTRING_LEN], test_str[ETH_GSTRING_LEN];
 
···
  * Fill in a block of loopback self-test entries.  Return new test
  * index.
  */
-int efx_fill_loopback_test(struct efx_nic *efx,
-			   struct efx_loopback_self_tests *lb_tests,
-			   enum efx_loopback_mode mode,
-			   unsigned int test_index, u8 *strings, u64 *data)
+static int efx_fill_loopback_test(struct efx_nic *efx,
+				  struct efx_loopback_self_tests *lb_tests,
+				  enum efx_loopback_mode mode,
+				  unsigned int test_index,
+				  u8 *strings, u64 *data)
 {
 	struct efx_channel *channel =
 		efx_get_channel(efx, efx->tx_channel_offset);
···
 	return n;
 }
 
-size_t efx_describe_per_queue_stats(struct efx_nic *efx, u8 *strings)
+static size_t efx_describe_per_queue_stats(struct efx_nic *efx, u8 *strings)
 {
 	size_t n_stats = 0;
 	struct efx_channel *channel;