net/mlx5e: RX, Generalize mlx5e_fill_mxbuf()

Make the function more generic. Let it get an additional frame_sz
parameter instead of deriving it from the RQ struct.

No functional change here, just a preparation for a downstream patch.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>