net/mlx5: separate Tx function declarations to another file

net/mlx5: separate Tx function declarations to another file

This patch separates Tx function declarations to different header file
in preparation for removing their implementation from the source file
and as an optional preparation for Tx cleanup.

Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>