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>