net/mlx5: E-switch, Fix printing wrong error value

When mlx5_modify_header_alloc() fails, instead of printing the error
value returned, current error log prints 0.

Fix by printing correct error value returned by
mlx5_modify_header_alloc().

Fixes: 6724e66b

 ("net/mlx5: E-Switch, Get reg_c1 value on miss")
Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>