net/mlx5: SF, silence an uninitialized variable warning

This code sometimes calls mlx5_sf_hw_table_hwc_init() when "ext_base_id"
is uninitialized.  It's not used on that path, but it generates a static
checker warning to pass uninitialized variables to another function.
It may also generate runtime UBSan  warnings depending on if the
mlx5_sf_hw_table_hwc_init() function is inlined or not.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>