xsk: fix return value of xdp_umem_assign_dev()

s/ENOTSUPP/EOPNOTSUPP/ in function umem_assign_dev().
This function's return value is directly returned by xsk_bind().
EOPNOTSUPP is bind()'s possible return value.

Fixes: f734607e ("xsk: refactor xdp_umem_assign_dev()")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>