mlx4: fix unneeded function error with clang 3.6

mlx4: fix unneeded function error with clang 3.6

When compiling with clang 3.6, the mlx4 driver gives the following error
message about an unneeded function.

  CC mlx4.o
.../drivers/net/mlx4/mlx4.c:136:20: fatal error: function
      'wr_id_t_check' is not needed and will not be emitted
[-Wunneeded-internal-declaration]
static inline void wr_id_t_check(void)
                   ^
1 error generated.

The function is to compile-time check the size of wr_id_t, so use
the standard DPDK BUILD_BUG_ON macro to do so in the init function
instead.

Fixes: 7fae69eeff13 ("mlx4: new poll mode driver")
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>