net/mlx5e: Correct snprintf truncation handling for fw_version buffer used by representors

snprintf returns the length of the formatted string, excluding the trailing
null, without accounting for truncation. This means that is the return
value is greater than or equal to the size parameter, the fw_version string
was truncated.

Link: https://docs.kernel.org/core-api/kernel-api.html#c.snprintf
Fixes: 1b2bd0c0

 ("net/mlx5e: Check return value of snprintf writing to fw_version buffer for representors")
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>