RDMA/mlx4: Copy union directly

RDMA/mlx4: Copy union directly

Copy union directly instead of using memcpy().

Note that in this case, a direct assignment is more readable and
consistent with the subsequent assignments.

This addresses the following -Wstringop-overflow warning seen in s390
with defconfig:
drivers/infiniband/hw/mlx4/main.c:296:33: warning: writing 16 bytes into a region of size 0 [-Wstringop-overflow=]
  296 |                                 memcpy(&port_gid_table->gids[free].gid,
      |                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  297 |                                        &attr->gid, sizeof(attr->gid));
      |                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

Link: https://github.com/KSPP/linux/issues/308
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/ZNvimeRAPkJ24zRG@work
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>