net/mlx5e: Put the common XDP code into a function

The same code that returns XDP frames and releases pages is used both in
mlx5e_poll_xdpsq_cq and mlx5e_free_xdpsq_descs. Create a function that
cleans up an MPWQE.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>