net/mlx5: fix build with clang 3.4.2

net/mlx5: fix build with clang 3.4.2

Compilation massage example:
"dpdk/drivers/net/mlx5/mlx5_flow_dv.c:1087:10: error: comparison of
unsigned enum expression < 0 is always false
[-Werror,-Wtautological-compare]
        if (reg < 0)
            ~~~ ^ ~
"

enum modify_reg holds only non-negative integers and in some places in
the code it was used to be compared with negative value, hence
compilation was failed.

Change all thus places to use integer instead of enum modify_reg.

Fixes: 3e8edd0ef848 ("net/mlx5: update metadata register ID query")
Fixes: 55deee1715f0 ("net/mlx5: extend flow mark support")
Cc: stable@dpdk.org
Signed-off-by: Matan Azrad <matan@mellanox.com>