net/mlx5: Fix false compilation warning

Fix the following warning:
drivers/net/ethernet/mellanox/mlx5/core//fs_core.c:845:5:
warning: 'err' may be used uninitialized in this function
[-Wmaybe-uninitialized]

No real issue here. This is only a false compiler warning.
The 'err' variable is guaranteed to be init by time of usage.

gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4)

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>