vdpa/mlx5: fix minsize build

vdpa/mlx5: fix minsize build

Error occurs when configuring meson with --buildtype=minsize
with GCC 11.1.0:

drivers/vdpa/mlx5/mlx5_vdpa_mem.c: In function ‘mlx5_vdpa_mem_register’:
drivers/vdpa/mlx5/mlx5_vdpa_mem.c:183:24: error:
initialization of ‘uint64_t’ {aka ‘long unsigned int’} from ‘void *’
makes integer from pointer without a cast [-Werror=int-conversion]
|         uint64_t gcd = NULL;
|                        ^~~~
drivers/vdpa/mlx5/mlx5_vdpa_mem.c:244:75: error:
‘mode’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
|                         klm_size = mode == MLX5_MKC_ACCESS_MODE_KLM ?
|                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|                               KLM_SIZE_MAX_ALIGN(empty_region_sz) : gcd;
|                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>