net/mlx: fix build with clang 9

net/mlx: fix build with clang 9

This rewrites the MKSTR macro appending an empty string to its arguments
to resolve build failures similar to:

  drivers/net/mlx4/mlx4.c:461:14: fatal error: format string is not a
  string literal [-Wformat-nonliteral]
          MKSTR(path, "%s/device/uevent", device->ibdev_path);

  drivers/net/mlx4/mlx4_utils.h:82:30: note: expanded from macro 'MKSTR'
          char name[snprintf(NULL, 0, __VA_ARGS__) + 1]; \

  drivers/net/mlx5/mlx5_stats.c:144:15: fatal error: format string is not a
  string literal [-Wformat-nonliteral]
  	MKSTR(path, "%s/ports/%d/hw_counters/%s",

  drivers/net/mlx5/mlx5_utils.h:149:30: note: expanded from macro 'MKSTR'
  	char name[snprintf(NULL, 0, __VA_ARGS__) + 1]; \

The errors reproduce with clang version 9.0.0, and the release notes
don't mention what could have caused them.

Fixes: 7fae69eeff13 ("mlx4: new poll mode driver")
Fixes: 771fa900b73a ("mlx5: introduce new driver for Mellanox ConnectX-4 adapters")
Cc: stable@dpdk.org
Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Signed-off-by: Matan Azrad <matan@mellanox.com>