net/mlx5e: Add missing increment of count

Add mistakenly missing increment of count variable when looping over
output buffer in mlx5e_self_test().

This resolves the issue of garbage values output when querying with self
test via ethtool.

before:
$ ethtool -t eth2
The test result is PASS
The test extra info:
Link Test        0
Speed Test       1768697188
Health Test      758528120
Loopback Test    3288687

after:
$ ethtool -t eth2
The test result is PASS
The test extra info:
Link Test        0
Speed Test       0
Health Test      0
Loopback Test    0

Fixes: 7990b1b5 ("net/mlx5e: loopback test is not supported in switchdev mode")
Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en_selftest.c

@@ -334,6 +334,7 @@ void mlx5e_self_test(struct net_device *ndev, struct ethtool_test *etest,
 		netdev_info(ndev, "\t[%d] %s start..\n", i, st.name);
 		buf[count] = st.st_func(priv);
 		netdev_info(ndev, "\t[%d] %s end: result(%lld)\n", i, st.name, buf[count]);
+		count++;
 	}
 
 	mutex_unlock(&priv->state_lock);